channels: channel/group description

[r10s]

we want to have at least a "channel" description, but when on that, it makes sense to directly go for a channel & group description. also UI wise, this is mostly the same.

the UI should show the description in the profile (similar to contact's status), maybe later also in otherwise empty chats.

• database-wise, we probably want in new column in the chats-table

• api-wise, things should be very similar to chat.get_name()/context.set_chat_name()

• as loading will be together with chat-object, we need both, a getter for cffi and a field for jsonrpc. for the setting, jsonrpc-only is fine.

• on the wire, idea is to do add a header-fields, and set that along with group-avatar changes (so, not always - but on member addition and changes)

once we have the api, it should be easy for UI to adapt.

ui should render the description as status and normal bubbles - so make links tappable etc.

(this issue is the outcome of various internal discussions)

discussions in the forum: https://support.delta.chat/t/add-topic-support-for-groups/261, https://support.delta.chat/t/a-resume-or-topic-for-groups/1342, https://support.delta.chat/t/group-description/2867, https://support.delta.chat/t/channel-description/4014

cc @Hocuri