Add topic support for groups

[adbenitez]

Just that, add an option to set the topic of a group, the topic should be show to new added users as a system message when they join the group

[Agus_H]

Indeed, would be very good to have a option to see topic of the group when you enter to one.

[Simon]

Link duplicate topic, so we can mark it as solved too when its implemented: A resume or topic for groups

[r10s]

good news: “group descriptions” are merged to core and will come with some of the next updates (probably not the 2.4x that is rolling already, but the next one) - stay tuned

allow to set chat description (#4214)