Code review comment for lp:~wallyworld/launchpad/better-popup-show-widget-name

I have modified the VocabularyPickerWidget to use FormattersAPI.css_id() to generate its input_id value.

The implementation of css_id has been changed:
1. '.' are considered valid id characters and will no longer be replaced by '-'. This is to allow ids like 'field.foo' to be rendered as is and not changed to 'field-foo'
2. If any invalid characters are found in the id and are replaced by '-', then the base64 encoding of the id name is appended to ensure uniqueness.

Unit tests have been added for css_id (there were none).

The only place that used css_id before this was the +review-licences page for projects. I checked that this page still works as expected.