Creating a project from a URL - only first URL is validated

[tfmorris]

The second and subsequent URLs are not validated before they're used to attempt to create a project (as discussed in #2534)

Steps to reproduce the behavior:

1. Go to 'Create Project' -> 'Web Addresses (URL)'
2. Paste a valid URL (e.g. "http://api.worldbank.org/countries/all/indicators/SP.POP.TOTL?date=2000:2001")
3. Click "Add another URL" and enter "badurl" (or any other invalid URL)
4. Click Next
5. See error modal dialog

Expected behavior

The value of all URL input fields should be validated to ensure that they are well-formed URLs.

OpenRefine (please complete the following information):

• master branch (3.4 snapshot)