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>

config/client.go

@@ -131,6 +131,7 @@ func GenerateClientConfig(c *model.Config, telemetryID string, license *model.Li
 	props["CollapsedThreads"] = *c.ServiceSettings.CollapsedThreads
 	props["EnableCustomGroups"] = "false"
 	props["InsightsEnabled"] = "false"
+	props["PostPriority"] = strconv.FormatBool(*c.ServiceSettings.PostPriority)
 
 	if license != nil {
 		props["ExperimentalEnableAuthenticationTransfer"] = strconv.FormatBool(*c.ServiceSettings.ExperimentalEnableAuthenticationTransfer)