[eviljeff]

Fixes mozilla/addons#16477

Description

Slightly larger scope than mozilla/addons#16477 - it attempts to address all urls that differ between pinguino and django devhub. The aim was to be able to flip back and forth between pinguino and old devhub without error pages.

Context

• The generic Coming Soon component serves as a useful proxy for pages we still need to implement
• as you can see in urls.py, there are a few urls that pingino uses that are inconsistent with django devhub, but they seemed like deliberate design choices, so added django redirects instead.
• there are some pages/content that we've merged into a single page - e.g. /developers/addons, /developers/themes, and /developers/feed are all on /developers now, so you can't flip twice to get back to the same page. I can think of a solution (e.g. including the old path as a GET param) but it seemed out of scope for this PR (which was already scope ballooned from just /addon/.../edit)
  • Added!
• in django we have RemoveSlashMiddleware but there's nothing equivalent in pinguino (yet) so if you add a / to the end of a url, it fails. We could fix this in a follow-up by adding a regex to the urls that optionally accepted /, or by reimplementing RemoveSlashMiddleware in django.
  • [Task]: Handle trailing slashes on urls in Pinguino addons#16548

Testing

• With the pinguino flag enabled, toggle in and out of pinguino on various pages
• All the pages should work without errors, and redirect where necessary
  • missing pages in pinguino should show Coming Soon

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.