MM-18356: Adds ability to paginate channel search. (#12830)

* MM-18356: Adds ability to paginate channel search.

* MM-18356: Minor refactor.

* MM-18356: Adds doc.

* MM-18356: Fixes doc.

* MM-18356: Some commentary, adds the total count to non-paginated responses, and removes a stray fmt.

* MM-18356: Fixes shadowed variable.

* MM-18356: Removes paginate field and API parameter.

* MM-18356: Adds method to check if channel search is a paginated request.

* MM-18356: Vet fix.

store/timer_layer.go

@@ -1592,20 +1592,20 @@ func (s *TimerLayerChannelStore) SaveMember(member *model.ChannelMember) (*model
 	return resultVar0, resultVar1
 }
 
-func (s *TimerLayerChannelStore) SearchAllChannels(term string, opts ChannelSearchOpts) (*model.ChannelListWithTeamData, *model.AppError) {
+func (s *TimerLayerChannelStore) SearchAllChannels(term string, opts ChannelSearchOpts) (*model.ChannelListWithTeamData, int64, *model.AppError) {
 	start := timemodule.Now()
 
-	resultVar0, resultVar1 := s.ChannelStore.SearchAllChannels(term, opts)
+	resultVar0, resultVar1, resultVar2 := s.ChannelStore.SearchAllChannels(term, opts)
 
 	elapsed := float64(timemodule.Since(start)) / float64(timemodule.Second)
 	if s.Root.Metrics != nil {
 		success := "false"
-		if resultVar1 == nil {
+		if resultVar2 == nil {
 			success = "true"
 		}
 		s.Root.Metrics.ObserveStoreMethodDuration("ChannelStore.SearchAllChannels", success, elapsed)
 	}
-	return resultVar0, resultVar1
+	return resultVar0, resultVar1, resultVar2
 }
 
 func (s *TimerLayerChannelStore) SearchForUserInTeam(userId string, teamId string, term string, includeDeleted bool) (*model.ChannelList, *model.AppError) {