Remove the (feature-flagged) view switcher component.

We missed the chance to strike while the iron was hot on this one -
neither Dawa or I have time anymore to work on further design
iterations. I don't want to have dead code living behind a feature flag
so I'm removing it. The code will still be in git if we ever want to
revive it. Perhaps these tabs can be re-designed as part of a larger
re-design of the sidebar in the future.

I'll leave the GitHub issue about the usability issues with the existing
selection tabs open: https://github.com/hypothesis/product-backlog/issues/327

For the record the main remaining issues with this were:

1. An undesirable visual popping happens when the view switcher loads on
pages that have a lot of annotations. This also happens with the
existing selection tabs but is arguably more visible with the view
switcher.

2. When there are only two tabs (no orphans) the visual design of the
tabs doesn't make it as immediately obvious which is the currently
selected tab. The other tab may be mistaken for the selected one.

The TODO list was here:

https://github.com/hypothesis/product-backlog/issues/327#issuecomment-311073581

The pull requests for this view switcher were:

https://github.com/hypothesis/client/pull/429
https://github.com/hypothesis/client/pull/481
https://github.com/hypothesis/client/pull/482
https://github.com/hypothesis/product-backlog/issues/327

Adding ADR for ePub support

Adding onLayoutChange documentation

Fix IE 11 regression in documents without a `<base>` element.

`Document#uri` and `Document#getMetadata` returned the base URI rather
than the document's real URL as the document location if the protocol of
the base URI and true document location were different. The reason for
this was to support the use case where the document is an iframe loaded
from a `blob:` URL, which Readium does for example.

This broke in IE 11 if the document did not have a `<base>` element
because the fallback `Node.baseURI` and `Document.documentURI`
properties used by `document-base-uri` do not exist in that browser.

This commit re-works the behavior of `_getDocumentHref` to be more
conservative and return the base URI only if a) the
document location does not have a whitelisted scheme and b) the base URI
is defined and does have a whitelisted scheme.

Additionally, the tests have been re-worked to test the public `uri`
method instead of the private `_getDocumentHref` method and use test
seams instead of proxyquire.

Use a URN derived from DC meta tags for creating equivalent doc links

“dc.relation.ispartof” is now used instead of “dc.source”
The goal of this is to identify a resource that’s part of another resource, therefore the more specific term fits better.

Use a URN derived from DublinCore meta tags for creating equivalent links across documents that provide ‘dc:source’ and ‘dc:identifier’ tags

This link is now the documentIdentifier for non-PDF cases for the document metadata. This metadata is shared with the sidebar, making this link appear in the list of links for the Search API calls

Adding support for onLayoutChange notifier configuration

- Add a doc comment to the _notifyOfLayoutChange function to clarify the
  purpose of the optional param.
- Add a comment inside the function to describe the high-level structure of the
  sidebar and how expanding/collapsing it is achieved.
- Correct a typo with "frameVisbileWidth"

Capture the client url to be used in subframes

When the client served by the browser extension injects itself into
child iframes on the current page, it needs to include the "assetRoot"
config setting in order for the client in the iframes to be loaded from
the extension rather than the default CDN URL.

Persist access and refresh tokens to localStorage.

Frame validity checks

As noted in PR feedback, the OAuth token key should be based on the
annotation service rather than just authority. The annotation service is
currently identified by (API domain, authority) but in future we'll
likely integrate the authority into the API URL for 3rd-party accounts
(eg.  "https://publisher.hypothes.is/api").

Rename for consistency with `saveToken`.

Convert search query parser to JS and add tests

Removing flags for multiple iframe support

For code in newly created JS files, arrow functions should be favored.

In the process fix an inconsistency where the "since" term was returned
as a number for hours, days, weeks, months and years but as a string for
seconds.

Convert the SearchFilter class to JS using decaffeinate and convert
methods which do not depend on any external state, which is all of them,
into plain functions.

Persist access and refresh tokens to localStorage, under the key:

  hypothesis.oauth.<authority>.token

Where `<authority>` is the domain of the annotation service and the
structure is the JSON-encoded object:

  {
    accessToken: <token>,
    refreshToken: <token>,
    expiresAt: <UNIX timestamp in ms>
  }

When an API token is requested for the first time during startup, the
existing token for the current authority is loaded from storage,
validated locally and then used if possible.