Validate that the input is a URL

Use a JavaScript version of John Gruber's URL validator to make sure
that what is inputted into the box is in fact a URL.

TODO: there is a BUG in that it accepts URL without an http:// prefix
      and we require that to be there.
Remove the form tag because I kept accidentally pressing enter

When you press enter, it submits the form and reloads the page which
is not what we want to have happen in this case. We want the add
button to be clicked (manually or programmatically).
Adopt a simpler URL validation regex

Gruber's would match any string as long as there was a valid URL
inside them, which is not what we want. Use a simpler regex that
matches URLs and only URLs.
Merge branch 'validate-url'