Add `embed` command group

imported from https://todo.sr.ht/~luma_inhibitor/cables/18

Command group to make custom embed posts through the bot. Users can either use the simple subcommand for a quick title and description, or use the json subcommand with a json file attached to fully specify the embed content.

Additionally, allow users to update embed posts with json containing fields to be updated. Unspecified fields in the json will remain unmodified in the embed post.

embed simple <title> <description>
embed json <+attachment.json>
embed edit <message> <+attachment.json>