Always serialize providers for Troupe

Almost all of the places where we serialize the troupe were already including providers. The providers are already present on the mongoose model so there are no performance implications (only the minuscule increase in payload size when sending json over to the client)

Not including providers can lead to errors (https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1742#note_276481842) since the SPA logic is expecting providers to always be in the roomMap in vuexStore.

Testing strategy

1. UserStrategy also accepts includeProviders, pay extra care in review to validate that the includeProviders attribute was only removed from TroupStrategy attributes
2. You can try to replicate the issue from https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1742#note_276481842 as a sanity check. (Not critical since the change has unit tests and it's very small)