add method to check if a repo/onto can be deleted
delete repository asynchronously
Use active scope for retrieving repositories.

I could have introduced a default scope, but this
would result in broken migrations: Some migrations
change existing repositories. They would fail at
selecting repositories because there is no column
'destroying'.

Also default scopes cannot be overridden.
Log destruction of repositories
Remove old mechanics to destroy repositories.

When destroying a repository asynchronously, we
don't need to mark it as destroying in a class
instance variable any more. Instead, we mark it
in the database.
fix code style according to hound comments
fiy typo
use lambda syntax for scopes
test the right function and improve error handling
refactor ontology delete view, use i18n and modal
add missing parenthesis
Revert "Use active scope for retrieving repositories."

This reverts commit d053813dcf22b74cded34daf5734e54388e9bee6.

Conflicts:
	app/controllers/home_controller.rb
	app/controllers/ontology_search_controller.rb
	app/helpers/application_helper.rb
	app/models/repository/destroying.rb
Add scopes and default scope, adjust migrations.
Fix errors introduced by default scope.
Merge branch '795-delete_repository_asynchonously' into develop