R1:95d6e0a3c49a

Store parents of forked pastes, and list child pastes if there are any.

Summary:
Added a 'parent' field which stores a PHID of another paste. If it is not NULL show a list of children pastes on view.
Also did some misc. refactoring to clean up the code a bit, specifically in the Create controller.

Test Plan:
- Checked old pastes, they were not affected.
- Added a paste, successfully.
- Forked it, successfully.
- Went to the original paste, saw the child paste listed.
- Forked it again, saw the new one…