MM-10140: API Implementation for Schemes related Endpoints (#8615)

* Implement basic scheme CRUD endpoints.

* Get All Schemes (Paged) Endpoint and store plumbing.

* Add get teams/channels for schemes.

* Fix unit tests.

* Review fixes.

* More review fixes.

store/sqlstore/channel_store.go

@@ -1730,7 +1730,7 @@ func (s SqlChannelStore) GetMembersByIds(channelId string, userIds []string) sto
 
 func (s SqlChannelStore) GetChannelsByScheme(schemeId string, offset int, limit int) store.StoreChannel {
 	return store.Do(func(result *store.StoreResult) {
-		var channels []*model.Channel
+		var channels model.ChannelList
 		_, err := s.GetReplica().Select(&channels, "SELECT * FROM Channels WHERE SchemeId = :SchemeId ORDER BY DisplayName LIMIT :Limit OFFSET :Offset", map[string]interface{}{"SchemeId": schemeId, "Offset": offset, "Limit": limit})
 		if err != nil {
 			result.Err = model.NewAppError("SqlChannelStore.GetChannelsByScheme", "store.sql_channel.get_by_scheme.app_error", nil, "schemeId="+schemeId+" "+err.Error(), http.StatusInternalServerError)