(Head) TAs assigned in other systems might need a job offer

If a TA is assigned as TA in another system (most commonly occurs for Head TAs), they do not have a job offer in TAM and thus will not get a contract. This should be fixed somehow.

• Show a message to generate an application if someone is already in the course as a (Head) TA
• Maybe distinguish between STUDENT and TEACHER

To do

• Some way of adding an (accepted) application manually