merge media and forum channels

right now forum and media channels are separate channels. however, their ui are very similar, and so are functionalities, with the exception that media channels forbid messages without attachments

i propose that the two channels merge into one. or at least, only disambiguated on the new channel modal with a toggle when the forum channel type is selected. alternatively, a new permission: send messages without attachments. then the channel icon would look at that permission for the forum channel for @everyone to decide the icon