Implement subproject/milestone conflict resolution rules

Summary:
Ref T10010. When you try to add "Sprint 35" to a task, remove "Sprint 34", etc. Briefly:

• A task can't be in Sprint 3 and Sprint 4.
• A task can't be in "A" and "A > B" (but "A > B" and "A > C" are fine).
• When a user makes an edit which would violate one of these rules, preserve the last tag in each group of conflicts.

Test Plan:

• Added fairly comprehensive tests.
• Added a bunch of different tags to things, saw them properly exclude conflicting tags.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10010

Differential Revision: https://secure.phabricator.com/D15167