Sometimes the context might hinder the anchoring if the context has
changed but the quote has not.

TextPositionAnchor can be generated and re-anchored within a PDF so
long as the selection doesn't span a page boundary.

TextQuoteAnchor is generated, but not re-anchored.

Rather than having each of the anchoring implementations have different
call signatures for their methods, use an options argument that varies
in content between the implementations, but probably shares some core
options like the root node.

The core of anchoring in the guest code is now asynchronous.

- Anchoring annotations happens asynchronously in the setup function.

- Creating an annotation is asynchronous, and depends upon the resolution
  of the targets, which may depend on the resolution of the document metadata.
  The first events fire only after this happens.

- The Anchor API can now optionally return Promises when converting ranges
  and selectors, though all the present implementations are synchronous.

It went down something like this:

:city_sunset::sparkles::alien::sweat_drops::pray::boom::city_sunrise:

s/viewable/visible/

Proper feature flags

Our first attempt at integrating feature flags with the frontend
required a rebuild and redeploy of the extension, which somewhat
defeats the purpose of feature flags. This commit wires up the new
feature client introduced in the preceding commits, allowing feature
toggles to update behaviour of already-deployed clients.

In order to respond to feature flag changes on the frontend, we need a
way of knowing what the current feature flag values are on the backend.

This commit adds a basic feature flag client that retrieves current
feature flag values by making an ajax request to the server. The client
also includes a cache, and supports retrieving new value from the server
when the cache expires.

Thread collapsing redux

Since there is no username, collapsing a draft when not signed in
had been causing the annotation to disappear completely. Now, there
is a clear prompt that holds the space.

Make the tags section use .annotation-body (because tags are a form of
body) and put the form actions, reply count, and license messages into
the footer so that these collpasing properly and uniformly.

This reverts commit 561544e4d289e728035aa9b549758b3b55d281f0.