Add an "importing" state to repositories and clean up the UI

Summary:
Fixes T3217. Ref T776. Ref T1493. Broadly, this introduces a mechanism which works like this:

• When a repository is created, we set an "importing" flag.
• After discovery completes, we check if a repository has no importing commits. Basically, this is the first time we catch up to HEAD.
• If we're caught up, clear the "importing" flag.

This flag lets us fix some issues:

• T3217. Currently, when you import a new repository and users have rules like "Email me on every commit ever" or "trigger an audit on every commit", we take a bunch of publish actions. Instead, implicitly disable publishing during import.
• An imported but un-pulled repository currently has an incomprehensible error on /diffusion/X/. Fix that.
• Show more cues in the UI about importing.
• Made some exceptions more specific.

Test Plan:
This is the new screen for a completely new repo, replacing a giant exception:

{F75443}

• Created a repository, saw it "importing".
• Pulled and discovered it.
• Processed its commits.
• Ran discovery again, saw import flag clear.
• Also this repository was empty, which hit some of the other code.

This is the new "parsed empty repository" UI, which isn't good, but is less broken:

{F75446}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, hach-que

Maniphest Tasks: T3607, T1493, T776, T3217

Differential Revision: https://secure.phabricator.com/D7429