Improve minor client behaviors for document rendering
Ref T13105. This adds various small client-side improvements to document rendering.
- In the menu, show which renderer is in use.
- Make linking to lines work.
- Make URIs persist information about which rendering engine is in use.
- Improve the UI feedback for transitions between document types.
- Load slower documents asynchronously by default.
- Discard irrelevant requests if you spam the view menu.
Test Plan: Loaded files, linked to lines, swapped between modes, copy/pasted URLs.
Maniphest Tasks: T13105
Differential Revision: https://secure.phabricator.com/D19256