Grab avatars from inside <a> tags.

Some mods/admins make this happen.
Fix parsing numeric post contents.

Same thing as with numeric usernames: comes as a JSON number, when we want a
string.
Stop marking posts as read from posts view.

Now that we reload the posts when marking as read, and now that posts no longer
have a "seen" property, we don't need all the marking posts machinery.
Remove unused junk.

Managed not to notice all this until now...
Move browser stuff to its own folder.
Add '(Un)Bookmark Thread' action to thread list.
Tap outside page picker to dismiss it.

Had some confusion about how to dismiss the page picker when accidentally
triggering it. Tapping the posts view seems logical enough.
Imgur API Test can upload multiple images.
Fix uploading multiple images.

Offsetting the range means:

  - We need to pull the key from the string related to the offset.
  - We don't need to offset the range again.

Fixing those two things worked wonders!
Formatting.
Awful Browser updates with dark mode setting.
Awful Browser's webview background color is white.

Web pages that don't specify a background color assume white, so we set it
after first load.
Awful Browser's toolbar looks more Awful-y.

Closes #172 (for now...)