Maintenance mode for Ontohub

Added capistrano tasks to disable the web interface and the Git shell for maintenance.
Merge branch 'staging' into 640-project_management

Conflicts:
	app/views/ontologies/edit.html.haml
Merge pull request #758 from ontohub/maintenance

Maintenance mode.
only admins can update and destroy metadata
Merge branch 'staging' into 640-project_management

Conflicts:
	app/models/ontology.rb
Merge branch '640-project_management' into develop