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.

model/channel.go

@@ -90,12 +90,17 @@ type DirectChannelForExport struct {
 // ExcludeDefaultChannels will exclude the configured default channels (ex 'town-square' and 'off-topic').
 // IncludeDeleted will include channel records where DeleteAt != 0.
 // ExcludeChannelNames will exclude channels from the results by name.
+// Paginate whether to paginate the results.
+// Page page requested, if results are paginated.
+// PerPage number of results per page, if paginated.
 //
 type ChannelSearchOpts struct {
 	NotAssociatedToGroup   string
 	ExcludeDefaultChannels bool
 	IncludeDeleted         bool
 	ExcludeChannelNames    []string
+	Page                   *int
+	PerPage                *int
 }
 
 func (o *Channel) DeepCopy() *Channel {