Dynamic completion for usernames (--assign, --assigned, --subscribe, --author)

Split out from #152.

Complete usernames from the API (`user.search`) for:

- `maniphest create/edit --assign`, top-level `edit --assign`
- `maniphest search --assigned`
- `--subscribe` on `maniphest create/edit`, `paste create/edit` and top-level `edit`, including the `@me` shortcut
- `paste search --author`

Things to decide: how to handle large user lists (constrain by what's been typed, see the `--tag` limit issue), and caching.

Start with issue #152 and trace the command entry points for the listed assign, assigned, subscribe, and author options, then inspect the `user.search` API usage. Define completion behavior for typed usernames, large result sets, caching, and the `@me` shortcut; done means all named commands provide dynamic username completion.

Written by the indexing model from the issue text.