Anchor hidden threads on the index.
Add Refreshed Navigation setting.
Close #1323, #1332.
Fix #1365.
Use the Last-Modified header's date for the index refresh timer.

Instead of showing how long has it been since we refreshed the index,
we show how long has it been since the index has changed on 4chan's side.
That'll be a more meaningful timer when 4chan freezes for example.
Changelog.
Merge pull request #1353 from MayhemYDG/index

[WIP] Index navigation improvements - EP03