Combine editing and adding sigs

Now uses a single view: edit_signature(). The add_signature() view
currently just calls the new version
Use new method for everything

urls have remained constant, reworked using the url paths to always use
the new method though
Remove unused view
Refactor bulk_import to not have to check whether sig is new or not
Merge pull request #36 from joshuablake/refactorSigUpdateUpstream

Refactor sig update