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Fix document metadata tests in chrome

Update Yarn lockfile following addition of cross-env in previous commit.

issue #659 make build platform independent by using cross-env

A `<link>` tag with a relative URL from a previous test was causing
problems in Chrome when testing the behavior of `Document#uri` in
documents with a non-HTTP/file `<base>`.

That is an edge case which we may want to cover at some point, but the
more significant issue is that the `Document#uri` tests were not
isolated from the earlier ones. Modify the `createDoc` helper so that it
uses a blank HTML document instead of using the global `document`
object.

The `Document` class for reading DOM document metadata takes an optional
DOM `Document` constructor argument as a test seam.

However the actual code only used the injected document in some places,
in others it used jQuery which in turn reads from the global
`window.document`.

Refactor this code to use the injected DOM Document everywhere and
remove the jQuery dependency.

Upgrade dependencies to fix compatibility with Node v10

Upgrade dependencies using `yarn upgrade`. This fixes dependency
installation with Node v10.

change links in help panel

"send a suppport ticket" now leads to `https://web.hypothes.is/get-help`
and "visit our help documents" leads to `https://web.hypothes.is/help`

Fix group focus change checks

The focused group change check had several issues:

 - The `prevFocused` variable was not updated after the focused group
   changed, so the `GROUP_FOCUSED` event was emitted every time the app
   state changed.

 - Groups were compared by reference rather than by ID, which means that
   a change which modified a property of the group but not its ID would
   cause the event to fire.

The first issue made it impossible to create annotations because the
unnecessary `GROUP_FOCUSED` event triggered a cascade of reactions which
crashed the app.

Modernize PDFMetadata class

 - Convert `PDFMetadata` to an ES2015 class

 - Convert the ad-hoc type documentation to tool-readable JSDoc

 - Add a little more detail in doc comments about what
   `documentFingerprint` is

Move focused group and list of loaded groups to store

As noted in CR feedback, having a selector named `focusedGroup` which
returns a `Group` and a state property named `focusedGroup` is
`string|null` is confusing.

Also change a `g` variable to `group` for clarity.

Also bring `yarn.lock` into sync

Remove outdated April fool joke.

Use group.links\.html instead of group\.url

Remove used used scss block causing invalid css error after upgrading…

Fixes: https://github.com/hypothesis/product-backlog/issues/542
Also see: https://github.com/hypothesis/product-backlog/issues/486

Add organization logos to groups menu

Resolve relative URLs when getting PDF URL from PDF.js

If PDF.js is launched with a `file` query-string parameter that is a
relative URL, then `window.PDFViewerApplication.url` will be that
relative URL.

When capturing the metadata from the PDF in order to search for
annotations or find relevant groups, we want to use an absolute URL.
This commit uses the existing `normalizeURI` helper to resolve the
possibly-relative URL.

Fixes https://github.com/hypothesis/product-backlog/issues/579