Meeting archiving: improvement for running process needed

[MSoeb]

Current behavior
When archiving meetings, there is no check as to whether there are ongoing processes, such as voting. This leads to practical problems. For example, accounts cannot be merged if they are in an ongoing voting process.

If a meeting is archived, the merge cannot simply be carried out. The current solution is to unarchive the meeting and end the voting. However, this (unarchiving) can only be done by a superadmin. This in turn leads to practical problems, as the superadmin is usually not immediately available.

Wanted behavior
To solve this problem, a check should be carried out during archiving to ensure that there are running processes. This includes votes on motions, elections and active speeches. If one of these processes is active, a dialog box should appear indicating that the following processes are active and should be terminated in order to perform archiving.

[rrenkert]

This should be handled in the backend I think. The client still needs to handle the "error" message.