Move member/watch actions to "Members/Watchers" page

Summary:
Ref T10054. This tries to make the members page a bit more consistent and provide hints to users about subproject/milestone membership rules. In particular:

• You now join, leave, watch, unwatch, add and remove members, and lock and unlock membership from the members screen.
• We now explain the membership rule for the project on this screen. There are currently four rules:
  • Normal Project: Join/leave normally.
  • Parent Project: Uses subprojects to determine members.
  • Milestone: Uses parent project to determine members.
  • Locked: Membership is locked.
  • (Future) Imported from LDAP/other external sources: Membership is determined by something else.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10054

Differential Revision: https://secure.phabricator.com/D15059