Add parameter to have note creation endpoint return full discussion JSON

We did a client side improvement to fix a performance issue happens when we add a comment into a big MR. To get the newly added discussion details we fetch all discussions then filter what we are looking for. This needs to be implemented in backend to avoid fetching all discussions.

/cc @smcgivern @felipe_artur

• @timzallmann commented on a discussion: (+1 comment)

  @timzallmann I think, our backend should also support this. There should be really no reason for us to fetch all of the discussions and then find we are looking for.

  However, I like the fix improvement here and I will merge this ASAP to make the GitLab.com better.

  Do we want to create a follow-up issue for backend improvement, or let's go with this approach?

  Totally agree, this was mainly a pragmatic doable fix. Yes please create one.