Before adding a meeting, make sure the calendar exists
Check is the request is GET to not overrides the form and be able to display the form errors
Add another calendar that is disabled
Rework checks for permission to add/edit a meeting

- When check is the user is an editor, return True if user is a calendar admin
- When a user tries to edit a meeting, block him/her if he/she is not an editor in that
calendar
Ignore a piece of code we should never hit (is_user_managing_in_calendar in add_meeting)
Simplify check, add another check when editing a meetin and ignore some pieces
Add tests for add_meeting, edit_meeting, auth_logout, my_meetings and more for auth_login