Allow for cloning/forking of submissions

[kgdunn]

Background

• Code snippets can be edited by other registered users at any time.
• Code packages and modules can only be edited by the submitter. Another user must first fork the package, with a different title and description to make changes. The original submission will be attributed.
• Code links to other sites can only be edited by the original submitter.

How it should work

Changes require a "change_comment" field to be filled in also.