Fix many encoding and architecture problems in Diffusion request and URI handling

Summary:
Diffusion request/uri handling is currently a big, hastily ported mess. In particular, it has:

• Tons and tons of duplicated code.
• Bugs with handling unusual branch and file names.
• An excessively large (and yet insufficiently expressive) API on DiffusionRequest, including a nonsensical concrete base class.
• Other tools were doing hacky things like passing ":" branch names.

This diff attempts to fix these issues.

• Make the base class abstract (it was concrete ONLY for "/diffusion/").
• Move all URI generation to DiffusionRequest. Make the core static. Add unit tests.
• Delete the 300 copies of URI generation code throughout Diffusion.
• Move all URI parsing to DiffusionRequest. Make the core static. Add unit tests.
• Add an appropriate static initializer for other callers.
• Convert all code calling newFromAphrontRequestDictionary outside of Diffusion to the new newFromDictionary API.
• Refactor static initializers to be sensibly-sized.
• Refactor derived DiffusionRequest classes to remove duplicated code.
• Properly encode branch names (fixes branches with "/", see https://github.com/facebook/phabricator/issues/100).
• Properly encode path names (fixes issues in D1742).
• Properly escape delimiter characters ";" and "$" in path names so files like "$100" are not interpreted as "line 100".
• Fix a couple warnings.
• Fix a couple lint issues.
• Fix a bug where we would not parse filenames with spaces in them correctly in the Git browse query.
• Fix a bug where Git change queries would fail unnecessarily.
• Provide or improve some documentation.

This thing is pretty gigantic but also kind of hard to split up. If it's unreasonably difficult to review, let me know and I can take a stab at it though.

This supplants D1742.

Test Plan:

• Used home, repository, branch, browse, change, history, diff (ajax), lastmodified (ajax) views of Diffusion.
• Used Owners typeaheads and search.
• Used diffusion.getrecentcommitsbypath method.
• Pushed a change to an absurdly-named file on an absurdly-named branch, everything worked properly.

{F9185}

Reviewers: nh, vrana, btrahan

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1921