Make it easier to configure an Asana workspace ID

Summary:
Ref T2852. It's a little tricky to figure out Asana workspace IDs right now. If the viewer has a linked account, just pull their workspaces and show them which IDs are available.

(In theory, we could use a `<select>`, but it would have more edge cases; this seems like a pretty solid fix.)

Test Plan: {F49938}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6437