Make mailinglist deletion safer/better informed

Ghost User
2019-11-20T13:12:51+01:00

### Motivation
Currently deleting a mailinglist will disable the group on gsuite, but delete the list on the website. This means that the name can be added as an alias for another list on the website, but gsuite will not support his, so we have a syncing error.

We could do an actual deletion on gsuite, and preserve archives another way. Or make sure the name cannot be used anymore after deletion, except when recreating the same list. If this method is chosen, the user should be informed on deletion.