Configure update checker on installation page (#21655)

- I recently became aware that this was enabled by-default, I don't
necessary agree with that and this should rather be configured by the
user(this patch does that on the installation page) as it connects to a
homeserver, which I'd prefer to avoid on my instance.


![image](https://user-images.githubusercontent.com/25481501/199260613-a77a1b10-347a-4542-8982-9b9b24dad28c.png)

routers/install/install.go

@@ -457,6 +457,7 @@ func SubmitInstall(ctx *context.Context) {
 	cfg.Section("service").Key("DEFAULT_ALLOW_CREATE_ORGANIZATION").SetValue(fmt.Sprint(form.DefaultAllowCreateOrganization))
 	cfg.Section("service").Key("DEFAULT_ENABLE_TIMETRACKING").SetValue(fmt.Sprint(form.DefaultEnableTimetracking))
 	cfg.Section("service").Key("NO_REPLY_ADDRESS").SetValue(fmt.Sprint(form.NoReplyAddress))
+	cfg.Section("cron.update_checker").Key("ENABLED").SetValue(fmt.Sprint(form.EnableUpdateChecker))
 
 	cfg.Section("").Key("RUN_MODE").SetValue("prod")