Use ApplicationTransactions and CustomField to implement build steps

Summary:
Ref T1049. Fixes T4602. Moves all the funky field stuff to CustomField. Uses ApplicationTransactions to apply and record edits.

This makes "artifact" fields a little less nice (but still perfectly usable). With D8599, I think they're reasonable overall. We can improve this in the future.

All other field types are better (e.g., fixes weird bugs with "bool", fixes lots of weird behavior around required fields), and this gives us access to many new field types.

Test Plan:
Made a bunch of step edits. Here's an example:

{F133694}

Note that:

  - "Required" fields work correctly.
  - the transaction record is shown at the bottom of the page.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4602, T1049

Differential Revision: https://secure.phabricator.com/D8600

resources/sql/autopatches/20140321.harbor.1.bxaction.sql

@@ -0,0 +1,21 @@
+CREATE TABLE {$NAMESPACE}_harbormaster.harbormaster_buildsteptransaction (
+  id INT UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
+  phid VARCHAR(64) NOT NULL COLLATE utf8_bin,
+  authorPHID VARCHAR(64) NOT NULL COLLATE utf8_bin,
+  objectPHID VARCHAR(64) NOT NULL COLLATE utf8_bin,
+  viewPolicy VARCHAR(64) NOT NULL COLLATE utf8_bin,
+  editPolicy VARCHAR(64) NOT NULL COLLATE utf8_bin,
+  commentPHID VARCHAR(64) COLLATE utf8_bin,
+  commentVersion INT UNSIGNED NOT NULL,
+  transactionType VARCHAR(32) NOT NULL COLLATE utf8_bin,
+  oldValue LONGTEXT NOT NULL COLLATE utf8_bin,
+  newValue LONGTEXT NOT NULL COLLATE utf8_bin,
+  contentSource LONGTEXT NOT NULL COLLATE utf8_bin,
+  metadata LONGTEXT NOT NULL COLLATE utf8_bin,
+  dateCreated INT UNSIGNED NOT NULL,
+  dateModified INT UNSIGNED NOT NULL,
+
+  UNIQUE KEY `key_phid` (phid),
+  KEY `key_object` (objectPHID)
+
+) ENGINE=InnoDB, COLLATE utf8_general_ci;