Add support for e-mail translations

I added a few things:
- I added the missing schema for the settings, that made the form to be translatable
- In the BookableCalendar entity, I set notification_email_subject, notification_email_body, notification_email_subject_cancel, notification_email_body_cancel to be translatable
- Also, I added the "Browse for available tokens" to both forms
- In Notification class, we detect the language to use in mail that way: if it is a valid user, use its preferred langcode. If anonymous, use the current site language. Also, I'm passing a bookable_calendar object to the token because if someone's enabled translation for the Calendar fields, we need to use the proper translation.
- also for a few tokens that may load referenced translatable entity, I added to code to use the exact language we need (if that does exist for that entity). Otherwise they loaded the original language only.

josh.fabean → made their first commit to this issue’s fork.

Tested that and everything seems to work as desired. Only change I made was change function _load_translated_entity to be namespaced to the module to not cause issues with other modules possibly.

In latest dev, will be in version 2.2.9 which should release today or tomorrow.