Automate URI Generation

[taineriley1]

@pierssinclairssw @william-liebenberg @bradystroud

This issue separates concerns in issue #465 and covers the URI generation.

In Netlify CMS, the URI field requires input from the user. It would be a better UX if this field was automatically populated with a value.

We will likely need a custom widget to do this (see https://www.netlifycms.org/docs/custom-widgets/). The URI field should still be editable. When the URI field is empty, the field should be filled with the rule's title separated by dashes and without punctuation. E.g. a title of 'Do you use great email signatures?' creates a URI of 'do-you-use-great-email-signatures'.

1. Add functionality to automate URI generation

AB#62677

[taineriley1]

A feature request in Netlify CMS's repo may provide functionality to implement this by enabling the use of placeholders in a widget's default field. See decaporg/decap-cms#1975