Implement "Repository" as a new-style CustomField in Differential

Summary:
Ref T3886. Ref T418.

• Adds new capabilities for CustomField:
  • Controls can now bulk-load PHIDs (e.g., for tokenizers).
  • Transactions can now bulk-load PHIDs (e.g., for relationship changes).
• Implements "Repository" control.
• Improves tokenizer StandardCustomField controls.

Test Plan:

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T418, T3886

Differential Revision: https://secure.phabricator.com/D8286