Move admin side article URL alias custom field validation to use pluggable_ui

**What problem does your feature solve?**
Currently browser side validation is done using a JS injection because it was easiest to do at the time (quick turn around, and was injecting CSS to support it anyway). However, this isn't really the best way to do this in Textpattern.

**Describe the solution you'd like**
Drop the JS injection to set the custom field pattern validation and replace it with a pluggable_ui call to modify the input control(s).