R1:778c970e312e

Modernize "mailable" typeahead datasources

Summary: Ref T4420. Modernize the mailing list datasource, then build a composite "mailable" datasource.

Test Plan:
- Edited "subscribers" field in Differential revision edit.
- Edited "subscribers" field in Differential search.
- Edited "add subscribers" field in differential revision view.
- Edited "add ccs" field in Diffusion commit view.
- Edited "add emails to CC" in a Herald rule.
- Edited "add ccs" in maniphest bulk editor.
- Edited "add ccs" in maniphest…