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/feature_flags.go

@@ -73,6 +73,8 @@ type FeatureFlags struct {
 	BoardsProduct bool
 
 	PlanUpgradeButtonText string
+
+	PostPriority bool
 }
 
 func (f *FeatureFlags) SetDefaults() {
@@ -100,6 +102,7 @@ func (f *FeatureFlags) SetDefaults() {
 	f.CallsEnabled = true
 	f.BoardsProduct = false
 	f.PlanUpgradeButtonText = "upgrade"
+	f.PostPriority = false
 }
 
 func (f *FeatureFlags) Plugins() map[string]string {