Remove chosen-js

[GuySartorelli]

There are a lot of problems with chosen-js, ranging from accessibility problems to difficulty customising the resulting markdown.

We should remove it and either rely on browser defaults or use a different library that provides better accessibility and customisation options.

Related

• Accessibility: Decorative icons should not be read by screen readers #1957 (comment)
• Accessibility problems in chosen dropdowns #1955
• Accessibility issues with Chosen harvesthq/chosen#264

[emteknetnz]

This should probably just happen as part of a wider "replace entwine with react" type of refactor (assuming that happens for CMS 7), otherwise we risk implementing a new jQuery based dropdown that will just end up getting replaced shortly there-after