Add optional multilingual support

:exclamation: this PR is based on seantis/seantis.reservation#34, please merge that one first.

This PR addresses the following issues:
* change the way UUIDs are retrieved from a `seantis.reservation.resource`. When `plone.multilingual` is installed UUIDs are taken from the translation-group that groups the same content in different languages. When `plone.multilingual` is not available the default UUID is used.
* Add translated labels for reservation forms.
* Fixes a problem with formatted ids in the calendars left-click menu