Put all DifferentialComment loading behind DifferentialCommentQuery

Summary:
Ref T2222.

I'm thinking about how I want to approach the Asana sync, and I want to try to do T2222 first so that we can build it cleanly on top of ApplicationTransactions. I think we can at least walk down this road a little bit and if it turns out to be scary we can take another approach.

I was generally very happy with how the auth migration turned out (seemingly, it was almost completely clean), and want to pursue a similar strategy here. Basically:

  - Wrap the new objects in the old objects for reads/writes.
  - Migrate all the existing data to the new table.
  - Everything hard is done; move things over a piece at a time at a leisurely pace in lots of smallish, relatively-easy-to-understand changes.

This deletes or abstracts all reads of the DifferentialComment table. In particular, these things are **deleted**:

  - The script `undo_commits.php`, which I haven't pointed anyone at in a very long time.
  - The `differential.getrevisionfeedback` Conduit method, which has been marked deprecated for a year or more.
  - The `/stats/` interface in Differential, which should be rebuilt on Fact and has never been exposed in the UI. It does a ton of joins and such which are prohibitively difficult to migrate.

This leaves a small number of reading interfaces, which I replaced with a new `DifferentialCommentQuery`. Some future change will make this actually load transactions and wrap them with DifferentialComment interfaces.

Test Plan: Viewed a revision; made revision comments

Reviewers: btrahan

Reviewed By: btrahan

CC: edward, chad, aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D6260
Add storage for new Differential transactions and transaction comments

Summary:
Ref T2222. Ref T1460. Depends on D6260.

This creates the new tables, but doesn't start using them. I added three new fields for {T1460}, to represent fixed/done/replied states.

Test Plan: Ran storage upgrade.

Reviewers: btrahan

Reviewed By: btrahan

CC: chad, aran

Maniphest Tasks: T1460, T2222

Differential Revision: https://secure.phabricator.com/D6261
Abstract access to DifferentialInlineComment behind a Query

Summary:
Ref T2222. See D6260.

Push all this junk behind a Query so I can move the storage out from underneath it.

Test Plan: Viewed home page, list view, revision. Made draft, looked at preview, submitted draft, viewed inline, replied to inline.

Reviewers: btrahan

Reviewed By: btrahan

CC: chad, aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D6262
Add LiskRawMigrationIterator

Summary: Ref T2222. I need this to migrate the Differential comment tables, because they are large. We have the similar `LiskMigrationIterator` already, but it won't work here because I intend to destroy the original objects after migrating them.

Test Plan:
Wrote a script to iterate over the `differential_comment` table, got reasonable output:

{P869}

Reviewers: btrahan

Reviewed By: btrahan

CC: chad, aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D6264
Allow date custom field to be blank

Summary:
Currently it's not allowed to be left blank (even with required: false)

Fixes T3343

Test Plan: Use the custom date field.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, chad

Maniphest Tasks: T3343

Differential Revision: https://secure.phabricator.com/D6251
Add PHIDs to DifferentialComments

Summary:
Ref T2222. This adds PHIDs to all Differential comments so I can migrate the inlinecommment table to transaction_comment in the next diff.

@wez, this will issue a few million queries for Facebook (roughly, one for each Differential comment ever made). It's safe to skip the `.php` half of the patch, bring Phabricator up normally, and then apply this patch with Phabricator running if that eases the migration, although the next few diffs will probably be downtime-required migrations so maybe it's easier to just schedule some downtime.

Test Plan: Ran migration locally. Verified existing comments and new comments received PHIDs.

Reviewers: btrahan

Reviewed By: btrahan

CC: wez, aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D6266
Make animations set their initial values immediately

Summary:
Currently, it takes one frame for animations to set their first value. For fading stuff in, that means it briefly appears at 100% opacity, then jumps to 0%, then fades in from there.

Instead, immediately tween to the initial value.

Test Plan: Comments in Pholio fade in nicely. Preview is still a janky pile of mess until D6267.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D6268