Add 'Bar Trained' role check when signing up for staffing

[mickzijdel]

Feature

• Check if member has the role 'Bar Trained' before letting them sign up for the 'Bar' or 'Bar Staff' role.
• Add badge for 'Bar Trained'

Fixes

• Fix issue with 'shows in previous academic years should not warn for non-members' test when executed the day before a change of academic year.