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.

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

services/forms/user_form.go

@@ -45,9 +45,6 @@ type InstallForm struct {
 	RegisterConfirm bool
 	MailNotify      bool
 
-	OfflineMode                    bool
-	DisableGravatar                bool
-	EnableFederatedAvatar          bool
 	EnableOpenIDSignIn             bool
 	EnableOpenIDSignUp             bool
 	DisableRegistration            bool