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 added to the list.

Reviewers:
epriestley

CC:

Differential Revision: 672