Comparison updates might involve some custom fetch process, so provide a
mechanism to register these via settings.py on a per-branch basis. If
an update command is defined for a branch and the logged-in user has the
new "update_comparison_branch" permission, an "Update" button will show
up on the recipes page for the comparison branch for authenticated
users that will trigger the command in the background (as a celery job)
and then show a page that displays the status. The status isn't shown in
real-time since that requires quite a lot of plumbing, but the page at
least auto-refreshes.
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
