MM-46408: adds PostPriority feature flag and admin setting (#20875)

* MM-46408: adds PostPriority feature flag

Adds the PostPriority feature flag. You can enable it by setting the
env variable `MM_FEATUREFLAGS_POSTPRIORITY=true`.

* Adds support for admin setting

* empty

* Uses post priority in telemetry

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

services/telemetry/telemetry.go

@@ -448,6 +448,7 @@ func (ts *TelemetryService) trackConfig() {
 		"enable_file_search":                                      *cfg.ServiceSettings.EnableFileSearch,
 		"restrict_link_previews":                                  isDefault(*cfg.ServiceSettings.RestrictLinkPreviews, ""),
 		"enable_custom_groups":                                    *cfg.ServiceSettings.EnableCustomGroups,
+		"post_priority":                                           *cfg.ServiceSettings.PostPriority,
 	})
 
 	ts.SendTelemetry(TrackConfigTeam, map[string]any{