bug 773522: Model fields to support concepts of max age and expiration for document rendering
bug 773522: Model method to render stale documents, either immediately or via delayed queue task
bug 773522: Account for one-shot render_expires without render_max_age for updates
bug 773522: Update go-tmux

* enable celeryd beat & events

* autoreload celeryd when source change detected

* tweak to not close panes & accept up-arrow when processes get Ctrl-C
bug 773522: Add rendering expiration celery tasks

* Celery task for rendering stale, with rate limit tweaks

* Add render_expires and render_max_age fields to Document admin

* render_expires is an optional field

* Optionally produce info log messages while rendering stale

* Enable celery queue by default in dev VM
bug 773522: Add render_max_age field to Revision and doc editing form
bug 773522: Quick management command for manually kicking off a round of rendering stale documents
bug 773522: Follow up with previous djcelery migration fix, returns control of migrations to app itself
bug 773522: Remove rate limit from render_stale_documents task
Merge pull request #1687 from lmorchard/773522-time-to-live

bug 773522: Support expiration date or max-age for rendered document content