feat: add notification for quota depletion

[miaulalala]

Fixes #8513

How to test

If your hoster doesn't support the QUOTA extension, you can replace the code in the background job with the following:

		$previous = 80;
		$quota = new Quota(91, 100);

//		$quota = $this->mailManager->getQuota($account);
//		if($quota === null) {
//			$this->logger->debug('Could not get quota information for account <' . $account->getEmail() . '>', ['app' => 'mail']);
//			return;
//		}
//		$previous = $account->getMailAccount()->getQuotaPercentage();

Current text:

To Do

• Test notifications ( I couldn't trigger them yet)
• Background jobs for existing accounts
• Tests

[st3iny]

I tested it using your proposed hack and it seems to work.

The notification entries are created in the notifications table but they don't show up in the web ui for some reason. I also attached a debugger to the cron to make sure that the notification is triggered.

[ChristophWurst]

Code looks good

[ChristophWurst]

IMO doing this less often would be fine too. The quota won't grow so rapidly. If it does, we won't catch it in time anyway.