Require URL field to be set when creating asset containers in CP

[gavinplatt]

Steps to reproduce

1. Create a new asset container via the CP
2. Leave URL field empty
3. Click Save

Expected behaviour

Should be prompted to enter a URL first (possibly using path, or name as default, similar to how slug works?)

Actual behaviour

Doesn't prompt - saves without URL being set.

Notes

This also applies to the Path field