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

config/default.json

@@ -186,6 +186,12 @@
         "BannerTextColor": "#333333",
         "AllowBannerDismissal": true
     },
+    "ThemeSettings": {
+        "EnableThemeSelection": true,
+        "DefaultTheme": "default",
+        "AllowCustomThemes": true,
+        "AllowedThemes": []
+    },
     "GitLabSettings": {
         "Enable": false,
         "Secret": "",