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

src/applications/diffusion/controller/DiffusionRepositoryCreateController.php

@@ -95,6 +95,10 @@ final class DiffusionRepositoryCreateController
           // transactions to apply this change.
           $repository->setCallsign($callsign);
 
+          // Put the repository in "Importing" mode until we finish
+          // parsing it.
+          $repository->setDetail('importing', true);
+
           $xactions[] = id(clone $template)
             ->setTransactionType($type_name)
             ->setNewValue(