anonymize collected IP addresses for GDPR

As a part of https://github.com/hypothesis/product-backlog/issues/653

Enable https://github.com/probot/stale

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