This is a pretty direct conversion of the `<annotation-viewer-content>`
component from AngularJS to Preact.

Much of the logic and tests are for non-UI logic, so could potentially
be moved to a service in future.

I encountered an odd issue with the `fetchThread` helper that required
it to be put at the bottom of the file. Putting it at the top resulted
in an error related to the `createElement` import at runtime.

An issue relating to the highlighting of which reply the ID in the URL
refers to is noted in the code. What should happen is that the reply
that the ID in the URL refers to is visually highlighted. This is an
existing regression from before this commit.