Move creation of knex instance to config module

addresses #2170
Only reference model properties through the models module.
This frees us up to enforce one single point of access, thus paving
the way towards allowing us to initialize the models at are request,
and not when it's require().

addresses #2170
Fix slowdown of tests by reverting back to using knex in test/utils
Merge pull request #3213 from hswolff/lazy-load-models

Preparation for lazy loading of models
Turn on update notifications for Ember admin

Issue #3160
- Use notifications API to display available update notification.
- Remove update_notification handlebars helper as now both the
  check for an available update and the notification handling
  is run from the server's admin controller index method.
- Bind the notification's location property to a css class
  for styling.
- Refactor Ember notifications to better handle notification
  objects.  Move responsibility for css class generation onto
  the notification component.
- Refactor gh-notifications component to take a location argument
  that's used to assign a css class and filter notifications.