[MM-41115] Add server-side username colorize preference (#19382)

* Server support for collapse consecutive preference

* Add server-side username colorize preference

model/preference.go

@@ -26,6 +26,8 @@ const (
 	PreferenceNameChannelDisplayMode      = "channel_display_mode"
 	PreferenceNameCollapseSetting         = "collapse_previews"
 	PreferenceNameMessageDisplay          = "message_display"
+	PreferenceNameCollapseConsecutive     = "collapse_consecutive_messages"
+	PreferenceNameColorizeUsernames       = "colorize_usernames"
 	PreferenceNameNameFormat              = "name_format"
 	PreferenceNameUseMilitaryTime         = "use_military_time"
 	PreferenceRecommendedNextSteps        = "recommended_next_steps"