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>

modules/setting/config/value.go

@@ -102,7 +102,7 @@ func (opt *Option[T]) ValueRevision(ctx context.Context) (v T, rev int, has bool
 	var valStr *string
 	if dynVal, hasDbValue := dg.GetValue(ctx, opt.dynKey); hasDbValue {
 		valStr = &dynVal
-	} else if cfgVal, has := GetCfgSecKeyGetter().GetValue(opt.cfgSecKey.Sec, opt.cfgSecKey.Key); has {
+	} else if cfgVal, hasCfgValue := GetCfgSecKeyGetter().GetValue(opt.cfgSecKey.Sec, opt.cfgSecKey.Key); hasCfgValue {
 		valStr = &cfgVal
 	}
 	if valStr == nil {