MM-52173 Add setting to delay channel autocomplete (#22952)

* Add unit tests for ChannelMentionProvider.handleCompleteWord

* Add a minimum length before the ChannelMentionProvider triggers

* MM-52173 Move delayed autcomplete behind a setting

* Fix e2e test type check

* Update Textbox tests

* Add new setting to server telemetry

server/model/config.go

@@ -973,6 +973,7 @@ type ExperimentalSettings struct {
 	EnableAppBar                    *bool   `access:"experimental_features"`
 	PatchPluginsReactDOM            *bool   `access:"experimental_features"`
 	DisableRefetchingOnBrowserFocus *bool   `access:"experimental_features"`
+	DelayChannelAutocomplete        *bool   `access:"experimental_features"`
 }
 
 func (s *ExperimentalSettings) SetDefaults() {
@@ -1015,6 +1016,10 @@ func (s *ExperimentalSettings) SetDefaults() {
 	if s.DisableRefetchingOnBrowserFocus == nil {
 		s.DisableRefetchingOnBrowserFocus = NewBool(false)
 	}
+
+	if s.DelayChannelAutocomplete == nil {
+		s.DelayChannelAutocomplete = NewBool(false)
+	}
 }
 
 type AnalyticsSettings struct {