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.