Improve minor client behaviors for document rendering

Summary:
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