Clean up AppURL, remove legacy origin-url webcomponent (#37090)

1. `origin-url` was introduced in the past when there was no good
framework support to detect current host url
    * It is not needed anymore
    * Removing it makes the code clearer
2. Separate template helper functions for different templates (web
page/mail)
3. The "AppURL" info is removed from admin config page: it doesn't
really help.
    * We already have various app url checks at many places

options/locale/locale_en-US.json

@@ -3224,10 +3224,8 @@
   "admin.config.server_config": "Server Configuration",
   "admin.config.app_name": "Site Title",
   "admin.config.app_ver": "Gitea Version",
-  "admin.config.app_url": "Gitea Base URL",
   "admin.config.custom_conf": "Configuration File Path",
   "admin.config.custom_file_root_path": "Custom File Root Path",
-  "admin.config.domain": "Server Domain",
   "admin.config.disable_router_log": "Disable Router Log",
   "admin.config.run_user": "Run As Username",
   "admin.config.run_mode": "Run Mode",