Add styling for URL Field

- Added `url-field.scss` to `ui-components`
- Defined styling to be used for an add on to the input field of the URL Field component. Intended for a `<span>` tag to wrap the Storage Selector directive in so it can sit to the right of the input field
- Updated `app.scss` to include `url-field`
Merge pull request #105 from Rise-Vision/chore/url-field-styling

Add styling for URL Field