R1:922e5c08497d

Projects - add "Additional Hashtags" to projects

Summary:
Fixes T4021. Chooses to keep a "primary" slug based off the name - including all that lovely logic - and allow the user to specify "additional" slugs. Expose these as "hashtags" to the user.

Sets us up for a fun diff where we can delete all the Project => Phriction automagicalness. In terms of this diff, see the TODOs i added.

Test Plan:
added a primary slug as an additional slug - got an error. added a slug in use on another project - got an…