Store and display CHANGELOGs

:fork_and_knife:

Using a similar approach to that which extracts READMEs, Supermarket now extracts CHANGELOGs from cookbook artifacts as they're uploaded. Cookbook versions with artifacts which contain a CHANGELOG will have a 'Changelog' tab on their detail page (to be specific: cookbook versions with artifacts uploaded _after_ this PR is deployed).
:+1: