bug 766252: Deferred and singleton page rendering

[lmorchard]

Dropping this bomb of a PR now and then stepping away from it for awhile. There's a lot here. I recommend reading all the commit log bullet points, and then review the tests before digging into any other code. This should run both with and without a Celery queue and celeryd running.

• Documents can be rendered on demand or offloaded into the Celery queue
  • Documents with defer_rendering == True are rendered with Celery
    queue tasks. Otherwise, rendering is attempted on demand during
    the request/reasponse cycle.
  • If a Document takes too long to render, defer_rendering is
    automatically set to True. This time limit is controlled by
    KUMA_DOCUMENT_FORCE_DEFERRED_TIMEOUT in Constance settings
• Allow only one rendering per Document at any given time. Additional
  attempts to render a Document while already in progress results in a
  warning message and/or stale content while the rendering finishes.
• Rendered content is now kept in the DB, rather than temporary cache
• Lots of additional Document fields to track rendering state, store
  rendered content, and control deferred rendering.
• Reload no longer triggers a render of the page. However, editing and
  saving a Document does.
• Shift-Reload by a logged-in user schedules a fresh render, assuming
  there's not one already in progress.
• Display warning messages about the render status of a page. But, only
  for logged-in users to keep search engine crawlers from indexing them.
• Display detailed Kumascript errors only to logged in users, also to
  avoid search indexing.
• Document rendering via kumascript refactored out of views.py and into
  models.py and kumascript.py
• Disable schedule_rebuild_kb and _maybe_schedule_rebuild in favor of
  new Document rendering system.
• Tweaks to Document admin to allow mass enable/disable of deferred
  rendering, link to public URL of Documents.
• Reenable celery queue in settings_local, tweaks to failing tests
• manage.py command for manually triggering or queuing a page render

[lmorchard]

Oh also, this should fix https://bugzilla.mozilla.org/show_bug.cgi?id=765649

[groovecoder]

should we add:

if not self.defer_rendering:
    return False

?

[lmorchard]

No. This function is about whether a rendering has been scheduled, which is valid whether or not the document should be rendered in the offline queue. (ie. A rendering is considered scheduled, even if it happens on-demand in the request/response cycle)

[lmorchard]

I just did a rebase with current master, and fixed the missing url in apps/wiki/templates/wiki/document.html