Bug 860865 Add All Downloads page for Beta, Aurora, and ESR to bedrock

Update the existing release channel page to support the other channels. This will require redirects from the old URLs, but I thought I would solicit feedback on the new urls first:

/firefox/all/ (same as current)
/firefox/beta/all/ (was /firefox/all-beta.html)
/firefox/aurora/all/ (was /firefox/all-aurora.html)
/firefox/organizations/all/ (was /firefox/organizations/all.html)

This PR doesn't attempt to merge all four channel "all" pages into one page with tabs or anything like that, though it may make sense to come back to that. This keeps them as distinct pages on the front-end, but shares a view/template on the backend.

I also wondered if I should be keeping all those conditionals out of the template, and creating child-templates for the beta/aurora/esr pages instead. Feedback please.