add sites from group interface

[johanpeters]

Currently the only way to add sites to a group is to edit every specific site and add them to a group.
it would be easier to have a option in the group configuration to edit / create groups and add one or multiple sites to the group. this would be a lot easer .

[nikosdion]

The group configuration would be the wrong place to do that. Conceptually, this belongs to sites management.

The only problem is that groups are configuration options, values inside a JSON field. I am not sure how efficient that would be, and whether it would cause issues such as timeouts. I have to take a closer look before committing to it.

[nikosdion]

Tracking with #480