Don't advertise support for messaging features that are disabled by flags

changed milestone to %Self-destructing messages

added Sponsor 6 label

But since we seem to have settled on the protocol design for image attachments, we can just signal support for it, but don't allow to send image attachments. So we only need to check for security critical bugs that can be caused by sending someone images.

resolved all threads

merged

mentioned in commit 03c55311