Allow any user to watch any project they can see

Summary:
Ref T6183. Ref T10054. Historically, only members could watch projects because there were some weird special cases with policies. These policy issues have been resolved and Herald is generally powerful enough to do equivalent watches on most objects anyway.

Also puts a "Watch Project" button on the feed panel to make the behavior and meaning more obvious.

Test Plan:

• Watched a project I was not a member of.
• Clicked the feed watch/unwatch button.

{F1064909}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T6183, T10054

Differential Revision: https://secure.phabricator.com/D15063