Restrict the list of requestable users by repo

[James-Firth]

This would mean people outside of a project won't get requested to review PRs they know nothing about.

[James-Firth]

Mergers/Managers/Admins should also be restricted to particular repos this will help with the Multi-Team Milestone

[James-Firth]

I suppose really each user should have a list of repos they care about and a perhaps a requestable, merger, and notifications flag for each, in addition to the global one.

So for instance when checking if I should send a user a message it should look something like

if (user.repos[thisRepo].notifications || user.global.notifications ) send a message

[KyleBrownCS]

I'll grab this