Move file prep to a separate method/action

so we can reuse (/forward to) it
Add /changes controller that shows file source

Fetch the file from the api and forward to source view.
Make not-found more interesting for /changes
Add Changes link to module side bar

Closes gh-499.
Closes gh-651.
Closes gh-827.
Remove change log detection; use /changes link
Test that source for Changes is displayed
Use pod view if change log is pod-formatted

Don't regress on #713
Test pod view of changes file
Test that 404 produces suggestion link
Use distribution link (over author/release) if latest
Order links consistently on module and release pages
Increase limit for searching root files

The only reason 'perl' didn't show a Changes link in the past
is because it has more than 200 files in the root.