Make Herald rules obey policies during application

Summary:
Ref T603. This closes the other major policy loophole in Herald, which was that you could write a rule like:

  When [Always], [Add me to CC]

...and end up getting email about everything. These rules are now enforced:

  - For a //personal// rule to trigger, you must be able to see the object, and you must be able to use the application the object exists in.
  - In contrast, //global// rules will //always// trigger.

Also fixes some small bugs:

  - Policy control access to thumbnails was overly restrictive.
  - The Pholio and Maniphest Herald rules applied only the //last// "Add CC" or "Add Project" rules, since each rule overwrote previous rules.

Test Plan:
  - Created "always cc me" herald and maniphest rules with a normal user.
  - Created task with "user" visibility, saw CC.
  - Created task with "no one" visibility, saw no CC and error message in transcript ("user can't see the object").
  - Restricted Maniphest to administrators and created a task with "user" visibility. Same deal.
  - Created "user" and "no one" mocks and saw CC and no CC, respectively.
  - Thumbnail in Pholio worked properly.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7224