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>

model/config.go

@@ -371,6 +371,7 @@ type ServiceSettings struct {
 	EnableSVGs                                        *bool `access:"site_posts"`
 	EnableLatex                                       *bool `access:"site_posts"`
 	EnableInlineLatex                                 *bool `access:"site_posts"`
+	PostPriority                                      *bool `access:"site_posts"`
 	EnableAPIChannelDeletion                          *bool
 	EnableLocalMode                                   *bool
 	LocalModeSocketLocation                           *string // telemetry: none
@@ -842,6 +843,10 @@ func (s *ServiceSettings) SetDefaults(isUpdate bool) {
 	if s.EnableCustomGroups == nil {
 		s.EnableCustomGroups = NewBool(true)
 	}
+
+	if s.PostPriority == nil {
+		s.PostPriority = NewBool(false)
+	}
 }
 
 type ClusterSettings struct {