fix maintenance mode for git

Ontohub::Application is not available in git mode,
as we do not preload the whole rails environment.
To get to related settings one needs the assume this:

Ontohub::Application.config ~= Settings.
fix file-extensions stubbing through :each

Stub file extensions on each spec run.
stub file extensions for all tests
Merge pull request #868 from ontohub/835-fix_ssh_again

fix maintenance mode for git
Merge branch 'staging' into 582-disable_editing_on_mirrors