R1:7ad68e63e49b

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…