[newgroups] add possibility to give a decline message for groups

[Thomas Niebler]

When declining a group request as an admin, add a possibility to give a reason why this group was rejected.

This might give admins better possibility to actually give a reason to users. It might also give a strange impression to spammers when their groups are rejected without an explicitly given reason.

[Andreas Hotho]

This is a good idea and needs to be added.

[Mario Holtmüller]

resolved #2447

→ <<cset 8bcd3957ae67>>

[Mario Holtmüller]

I implemented it within the existing table. Hope you like it this way.