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>

models/user/avatar.go

@@ -74,7 +74,7 @@ func (u *User) AvatarLinkWithSize(ctx context.Context, size int) string {
 	switch {
 	case u.UseCustomAvatar:
 		useLocalAvatar = true
-	case disableGravatar, setting.OfflineMode:
+	case disableGravatar:
 		useLocalAvatar = true
 		autoGenerateAvatar = true
 	}