Use GlFormInvalidFeedback component in Fork Form

Let's ensure invalid feedback is provided for our project URL in the Fork Form

More Info

https://bootstrap-vue.org/docs/components/input-group#input-groups

Bootstrap v4 currently does not support contextual state styling (i.e. valid or invalid) of input groups. However, the inputs inside the input group do support contextual state.

So we end up having something like this:

GlFormInputGroup	Other form input (for comparison)

Because of this problem, we are introducing a new PJ component GlFormInvalidFeedback (Issue > gitlab-ui#1255 (closed)) This will allow us to circumvent the bootstrap v4 concern. And it was also the suggested solution from Bootstrap Vue > https://github.com/bootstrap-vue/bootstrap-vue/issues/1251#issuecomment-340065341.