Modernize "repositories" typeahead datasource

Summary:
Ref T4420.

  - Allow tokenizers to accept either a `Datasource` object (new style) or a URI (old style).
  - Read URI and placeholder text from object, if available.
  - Swap the "repositories" datasource (which seemed like the simplest one) over to the new stuff.
  - Tweak/update the repo tokens a little bit.

Test Plan:
  - Used tokenizer in Herald, Differential (search), Differential (edit), Push Logs.
  - Grepped for other callsites.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4420

Differential Revision: https://secure.phabricator.com/D9874