R1:34628002fd96

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…