Improve some Forms (#24878)

Don't really know a better name for this. I've gone through some Forms
and added missing HTML attributes (mostly `maxlength`). I tried to fill
the Forms with dummy Data and see if Gitea throws a Error (e.g. maximum
length). If yes, I added the missing HTML attribute.

While working on this, I discovered that the Form to add OAuth2 Apps
just silently fails when filled with invalid data, so I fixed that too.

routers/web/user/setting/oauth2_common.go

@@ -31,6 +31,7 @@ func (oa *OAuth2CommonHandlers) renderEditPage(ctx *context.Context) {
 func (oa *OAuth2CommonHandlers) AddApp(ctx *context.Context) {
 	form := web.GetForm(ctx).(*forms.EditOAuth2ApplicationForm)
 	if ctx.HasError() {
+		ctx.Flash.Error(ctx.GetErrMsg())
 		// go to the application list page
 		ctx.Redirect(oa.BasePathList)
 		return