Organizations index should be CclaSignatures json index

:fork_and_knife: Since organizations index actually returns `CclaSignature`s what is now the organizations index should be the json format of the `CclaSignaturesController`. This feature now also now uses the `CclaSignature.by_organization` scope so the `CclaSignature`s returned are unique by organization, this should resolve any current confusion surrounding this feature.

_This should address the last two QA items on [this card](https://trello.com/c/VrGr5RM4/211-allow-supermarket-admins-to-combine-ccla-organizations), mainly just the use of `CclaSignature.by_organization` addresses these issues. Moving the action to the CclaSignatures isn't related to the bug but it seemed to make more sense and it allows us to use the same scope call that's already being used for the HTML response._