Fix /tag/aa%20bb project URIs

Summary:
Ref T9551. To set things up:

• Name a project aa bb. This will have the tag aa_bb.
• Try to visit /tag/aa%20bb.

Here's what happens now:

• You get an Aphront redirect error as it tries to add the trailing /. Add phutil_escape_uri() so that works again.
• Then, you 404, even though this tag is reasonably equivalent to the real project tag and could be redirected. Add a fallback to lookup, resolve, and redirect if we can find a hit for the tag.

This also fixes stuff like /tag/AA_BB/.

Test Plan: Visited URIs like /tag/aa%20bb, /tag/aa%20bb/, /tag/Aa_bB/, etc. None of them worked before and now they all do.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9551

Differential Revision: https://secure.phabricator.com/D14260