Storage limit: 80% warning and limit in the web interface

- after each successful backup the stored size is recorded; at 80% of a
  location's limit administrators get a notification (once a day)
- locations list shows stored size and limit; the add form has a
  'maximum size' field that explains that empty means unlimited and
  billed by the provider; PUT /api/targets/{id}/limit (password
  confirmation)

Tested on NC34 with the OVH test bucket: a backup above the limit stops,
a successful one at 81% sends the warning.

lib/Service/BackupService.php

@@ -149,6 +149,7 @@ class BackupService {
 				$state['files'] = BackupRun::step($repo, $state['files'], $deadline);
 				if ($state['files']['phase'] === 'done') {
 					$this->recordSnapshot($run, $state['files']);
+					$this->alerts->storedBytes($target, $repo->storedBytes());
 					$run->setStatus(Run::DONE);
 					$run->setPhase('done');
 					$run->setFinishedAt(time());