Why:
Somehow we're seeing communities without proposals at production. We
must find why and fix it, but first we need to throw a 404 at the user
instead of a 500 internal server error
How:
First catching the scenario of non-existent communitable at the
controller and raising a 404 error. Secondly preventing the author_id
access over a possibly nil object, this is a smell but it can't be
easily fixed right now... we need to correctly implement a relation
between Community and communitable and avoid the multiple occurences of
`community.from_proposal?` in the codebase that makes it impossible to
extend to a fourth communitable model.
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