[NEW] Rest endpoints of discussions

[MarcosSpessatto]

Closes #13973

[engelgabriel]

Can we please keep consistency between DDP and REST API calls?

[rodrigok]

@ggazzo @rafaelks @engelgabriel is necessary to have a endpoint to list discussions? Aren't they part of rooms?

[rafaelks]

@rodrigok Yes, you can be a non member of a discussion but still want to access it.

[rodrigok]

@rafaelks ok, so then explain how this endpoint would be used. Where are you planing to list the discussions of a single room?

[rafaelks]

@rodrigok Like the web client, the mobile apps could display all discussions of a Room, similar to Attachments, Pinned, Starred, etc. What do you think?

[rodrigok]

@rafaelks ah, ok, sure, didn't remember we had a place to list discussions of a single room, it makes sense now.

[engelgabriel]

@rafaelks is correct, we have this on the web, the user can use the sidebar to see all discussions of a room