Don't accept invalid URLs

[WilliamWDTK]

See the history of https://www.whatdotheyknow.com/admin/bodies/1001

An invalid URL (banana) for disclosure log was accepted, and led to https://www.whatdotheyknow.com/body/banana.

I perhaps should not have tested it on a live, body, but it has now been reverted.

[FOIMonkey]

+1 I've accidentally pasted tags into the homepage field before, which this would've prevented.

[RichardTaylor]

This would tackle the case of accidentally putting an email address into these fields.

It leads me to wonder about further validation eg. rejecting a notes or tags field containing just a URL or email address.

Also consider if we want validation on the CSV upload or just the admin create/edit form.

[WilliamWDTK]

I'd say ideally on CSV too, but I imagine that would increase timeouts on WDTK, which is a separate issue.