Enable the import_url field.

Closes #28531
The field should be enabled on `project/import/new`.
It shouldn't hurt to have the field enabled on `projects/new` as well.

app/views/shared/_import_form.html.haml

@@ -2,7 +2,7 @@
   = f.label :import_url, class: 'control-label' do
     %span Git repository URL
   .col-sm-10
-    = f.text_field :import_url, autocomplete: 'off', class: 'form-control', placeholder: 'https://username:password@gitlab.company.com/group/project.git', disabled: true
+    = f.text_field :import_url, autocomplete: 'off', class: 'form-control', placeholder: 'https://username:password@gitlab.company.com/group/project.git'
 
     .well.prepend-top-20
       %ul