Add "Flags" to allow users to collect the things they love

Summary:
Flags are a personal collection of things you want to take a look at later. You can use several different colors and add notes.

Not really sure if this is actually a good idea or not but it was easy to build.

Planned features:

• Allow Herald rules to add flags.
• In the "edit flag" dialog, have a "[x] Subscribe Me" checkbox that CCs you.
• Support Diffusion.
• Support Phriction.
• Always show flags on an object if you have them (in every view)?
• Edit dialog feels a little heavy?
• More filtering in /flag/ tool.
• Add a top-level links somewhere?

Test Plan: Added, edited and removed flags from things. Viewed flags in flag view.

Reviewers: aran, btrahan

Reviewed By: btrahan

CC: aran, epriestley, Koolvin

Maniphest Tasks: T1041

Differential Revision: https://secure.phabricator.com/D2024