R1:3ff5ca789a60

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.