Create an admin group for a calendar

This group will be allowed to edit and delete any meeting from this calendar but none
other.
Add the alembic migration for the calendar_admin_group field in the calendar table
Fix typo in the Add calendar form
Add and display the calendar_admin_groups field in the add and edit calendar forms
Admins of a calendar are considered as managers as well

This gives admin the rights to edit/delete any meetings of this calendar but also to create
some
For a meeting to be recursive it needs a frequency and a end date
When searching for the meeting of someone, make sure there is the comma after the nick

Otherwise the match is too large and it gives to pingou rights on meeting created by
pingou2.

Ajust unit-test accordingly
Implement the calendar admin

This admin has full power over a single calendar as opposed to the fedocal admins that
have full power over all the calendars.
The admin calendar are defined in the calendar table and correspond to FAS groups.