Added `recipientUsers` include for `CreateDiscussionController`

[iPurpl3x]

Changes proposed in this pull request: Added recipientUsers include for CreateDiscussionController. This change has no direct effect on this extension but it fixed errors that other extensions could have because there is a discussion created in a custom way and the recipientUsers are not serialized, the following error might occur (see line of code):

TypeError: e.recipientUsers().find is not a function
    Pt Discussion.js:98

Reviewers should focus on: Nothing is broken and that this change doesn't impact performance

Confirmed

• [x] Frontend changes: tested on a local Flarum installation.
• [x] Backend changes: tests are green (run composer test).

[imorland]

Thanks @iPurpl3x - this makes sense to include