Optimise creation of DM (#16819)

* Optimise creation of dm

* Handle direct channels with the same user

* Cover GetMany with specs and add it on tha cache layer as well

* Fix specs by handling user dming themselves

* Apply PR suggestions

* Apply PR suggestions

* Use require.NoError instead of require.Nil on userstore test

* Improve readability of GetOrCreateDirectChannel

* Apply PR suggestions

* Update layers

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

store/store.go

@@ -323,6 +323,7 @@ type UserStore interface {
 	UpdateMfaSecret(userId, secret string) error
 	UpdateMfaActive(userId string, active bool) error
 	Get(id string) (*model.User, error)
+	GetMany(ids []string) ([]*model.User, error)
 	GetAll() ([]*model.User, error)
 	ClearCaches()
 	InvalidateProfilesInChannelCacheByUser(userId string)