Configs for themes in Display Settings: hide themes options, hide custom themes, specific list of themes (#7173)

* Add configuration to enable or disable choosing themes in Display Settings. Only for Licensed servers.

* Add configuration to enable or disable choosing custom themes in Display Settings. Only for Licensed servers.

* Add configuration to enable or disable a specific list of themes to choose in Display Settings. Only for Licensed servers.

* Added config value and logic for "DefaultTheme"

* Fix problem with undefined values when the server is not licensed

model/license.go

@@ -51,6 +51,7 @@ type Features struct {
 	PasswordRequirements      *bool `json:"password_requirements"`
 	Elasticsearch             *bool `json:"elastic_search"`
 	Announcement              *bool `json:"announcement"`
+	ThemeManagement           *bool `json:"theme_management"`
 	EmailNotificationContents *bool `json:"email_notification_contents"`
 
 	// after we enabled more features for webrtc we'll need to control them with this
@@ -152,6 +153,11 @@ func (f *Features) SetDefaults() {
 		*f.Announcement = true
 	}
 
+	if f.ThemeManagement == nil {
+		f.ThemeManagement = new(bool)
+		*f.ThemeManagement = true
+	}
+
 	if f.EmailNotificationContents == nil {
 		f.EmailNotificationContents = new(bool)
 		*f.EmailNotificationContents = *f.FutureFeatures