Use "Enable Gravatar" but not "Disable" (#36771)

* Fix #35685
* Fix #35627
* Fix #31112


Introduce "fipped" config value type, remove unused setting variables.
Make DisableGravatar=true by defult, remove useless config options from
the "Install" page.

The legacy config options are still kept because they are still the
fallback values for the system config options.

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>

options/locale/locale_en-US.json

@@ -285,12 +285,6 @@
   "install.register_confirm": "Require Email Confirmation to Register",
   "install.mail_notify": "Enable Email Notifications",
   "install.server_service_title": "Server and Third-Party Service Settings",
-  "install.offline_mode": "Enable Local Mode",
-  "install.offline_mode_popup": "Disable third-party content delivery networks and serve all resources locally.",
-  "install.disable_gravatar": "Disable Gravatar",
-  "install.disable_gravatar_popup": "Disable Gravatar and third-party avatar sources. A default avatar will be used unless a user locally uploads an avatar.",
-  "install.federated_avatar_lookup": "Enable Federated Avatars",
-  "install.federated_avatar_lookup_popup": "Enable federated avatar lookup using Libravatar.",
   "install.disable_registration": "Disable Self-Registration",
   "install.disable_registration_popup": "Disable user self-registration. Only administrators will be able to create new user accounts.",
   "install.allow_only_external_registration_popup": "Allow Registration Only Through External Services",
@@ -3193,7 +3187,6 @@
   "admin.config.custom_conf": "Configuration File Path",
   "admin.config.custom_file_root_path": "Custom File Root Path",
   "admin.config.domain": "Server Domain",
-  "admin.config.offline_mode": "Local Mode",
   "admin.config.disable_router_log": "Disable Router Log",
   "admin.config.run_user": "Run As Username",
   "admin.config.run_mode": "Run Mode",
@@ -3296,7 +3289,7 @@
   "admin.config.cookie_life_time": "Cookie Life Time",
   "admin.config.picture_config": "Picture and Avatar Configuration",
   "admin.config.picture_service": "Picture Service",
-  "admin.config.disable_gravatar": "Disable Gravatar",
+  "admin.config.enable_gravatar": "Enable Gravatar",
   "admin.config.enable_federated_avatar": "Enable Federated Avatars",
   "admin.config.open_with_editor_app_help": "The \"Open with\" editors for the clone menu. If left empty, the default will be used. Expand to see the default.",
   "admin.config.git_guide_remote_name": "Repository remote name for git commands in the guide",