Add modal for adding mentions to the text

[merlinfuchs]

Add a button to the markdown controls that opens a modal where you can enter a user id, channel id, role id, or timestamp and it will automatically insert the mention in the right format.

[merlinfuchs]

This could also give you a list of channels, roles, and even members of the server to select from. Caching members isn't doable for all servers, so this could be a premium feature.

[merlinfuchs]

This could also include a datetime picker for timestamps in markdown