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>

custom/conf/app.example.ini

@@ -238,9 +238,6 @@ RUN_USER = ; git
 ;; Indicate whether to check minimum key size with corresponding type
 ;MINIMUM_KEY_SIZE_CHECK = false
 ;;
-;; Disable CDN even in "prod" mode
-;OFFLINE_MODE = true
-;;
 ;; TLS Settings: Either ACME or manual
 ;; (Other common TLS configuration are found before)
 ;ENABLE_ACME = false
@@ -1983,12 +1980,12 @@ LEVEL = Info
 ;; or a custom avatar source, like: http://cn.gravatar.com/avatar/
 ;GRAVATAR_SOURCE = gravatar
 ;;
-;; This value will always be true in offline mode.
+;; Deprecated, see Web UI Admin Panel -> Config -> Settings
 ;DISABLE_GRAVATAR = false
 ;;
 ;; Federated avatar lookup uses DNS to discover avatar associated
 ;; with emails, see https://www.libravatar.org
-;; This value will always be false in offline mode or when Gravatar is disabled.
+;; Deprecated, see Web UI Admin Panel -> Config -> Settings
 ;ENABLE_FEDERATED_AVATAR = false
 
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