`externalContainerSelector` config implementation

Fix various failures when running test suite under Chrome

Fix 404 warnings from headless Chrome during tests caused by trying to
load non-existent image URLs in the anchoring regression tests.

The previous method of setting the `__hypothesis_frame` property on the
Window did not work. I haven't investigated in detail why, but the
alternate method of setting this property used here works instead.

Fix adder toolbar tests in browsers (eg. Chrome) which natively support
Shadow DOM v1. The test for Shadow DOM v0 failed because the implementation
feature-detected and used the Shadow DOM v1 method
`Element.prototype.attachShadow` instead.

Fix the issue by patching the test element to hide the native Shadow DOM
methods if they exist.

`uri` is a mandatory property of actual annotations. The absence was
causing some tests to fail in Chrome due to the URL constructor being
called with `undefined`.

`toLocaleTimeString()` in Chrome produces an `hh:mm:ss` output on my
system whereas the rendered timestamp only has `hh:mm`.

Re-using `format` from date-util avoids this hazard.

Add missing arguments to `Element#setAttribute` and
`Selection#collapse`.

PhantomJS allowed these missing arguments but other browsers, eg.
Chrome, do not.

Convert PdfSidebar to JS

Fix "No results for {query}" message

Usage of `vm.search.query` in the `<sidebar-content>` template was
incorrect. `vm.search` is always set and `query` is a function that
returns a string, not a string.

Fixes #9

Handle non percent-encoded URLs when replacing links with media embeds

When a user pastes a link to a video or audio file in an annotation,
handle the case where the link contains non-ASCII characters but the
link's text has not been percent-encoded.

In that case the link's text content will be the unencoded form of the
URL, but the `link.href` property will return the percent-encoded form,
so the two do not compare equal and therefore the link was not replaced.

ES 6 does not provide a direct equivalent for CoffeeScript's static
class properties so the initialization of `options` has been refactored
to use `Object.assign()` as a more idiomatic way to combine the config
args passed to the constructor with the defaults for `PdfSidebar`.

- Set a new `externalFrame` property when the external container config is provided.
- Use the new instance var is over the existing `frame` var.
	- This was done to isolate the new usage from the existing usage of `frame`
	- The existing usage of `frame` applies control over the sidebar frame, which works against what we want with `externalFrame`
- Make the uses of `frame` conditional.
	- These are the statements that are not invoked when the external container is specified.
	- The one case where we do want to use either frame is when emitting `onLayoutChange`
- Refactor the Sidebar tests to test for the added implementation and usage

- Add logic that removes the plugins conditionally based on the config.
- Move the existing logic for the `theme` config to be grouped with the above.
- The new location for the existing logic is better for testing.
- Add tests for the new and existing logic.

Convert annotator/plugin/document.coffee to JS

anonymize collected IP addresses for GDPR

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

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

Shorten very long lines produced by decaffeinate

Clean up output from `decaffeinate`.

 - Remove unnecessary `return` statements
 - Rename `Document` to `DocumentMeta` to avoid conflict with global
   `Document` and match the name used in `document.js`
 - Disable ESLint bind warnings. The use here is appropriate.

Clean up various aspects of `decaffeinate`'s conservative output.

 - Rename `Document` class to `DocumentMeta` to avoid conflict with
   `Document` global
 - Remove class wrapper
 - Remove method `this` binding
 - Remove unnecessary `Array.from` calls
 - Remove unnecessary `return` statements

Initial automated CoffeeScript => JS conversion using:

```
decaffeinate <CoffeeScript file>
eslint --fix <JS file>
```

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