bookable_resources 1.0.0-alpha5

Reminders now work on their own. A site can send reminders before a booking without enabling moderation or notifications.

### New submodules

- **Bookable Resources Mail** sends every email about a reservation, in each recipient's language, and logs it. It provides the `bookable_email` text format, the plain-text fallback and the **Email log** (Configuration › Email log), with its retention under **Email settings**.
- **Bookable Resources Tokens** provides the `[bookable_resources:*]` tokens and the token list shown beside the email forms.

### Changes

- New service `bookable_resources.reservation_approval` (`ReservationApprovalInterface::isApproved()`). Without moderation every reservation is approved; moderation decorates it to read the reservation's status.
- Reminders depend on Mail and Tokens only. Notifications depend on Moderation, Mail and Tokens, and send through Mail.
- The notification log is now the Email log (`bookable_mail_log`). Every email leaves under the module `bookable_resources_mail` with the mail key `reservation`. The Notification settings page is gone; its retention setting moved to Email settings.
- Help: new topics "Emails about reservations", "Emails on the server" and "Tokens in emails". The developer topics document the new service, the tokens and `MailSender`.