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>

routers/web/admin/config.go

@@ -126,7 +126,6 @@ func Config(ctx *context.Context) {
 	ctx.Data["AppUrl"] = setting.AppURL
 	ctx.Data["AppBuiltWith"] = setting.AppBuiltWith
 	ctx.Data["Domain"] = setting.Domain
-	ctx.Data["OfflineMode"] = setting.OfflineMode
 	ctx.Data["RunUser"] = setting.RunUser
 	ctx.Data["RunMode"] = util.ToTitleCase(setting.RunMode)
 	ctx.Data["GitVersion"] = git.DefaultFeatures().VersionInfo()