Refactor CSS/JS

[djowett]

content-chooser.css should be authenticated users only, however it has some rules referencing more-link-field. (This is a follow up to #750)

Other folk @hvelarde @idgserpro may have more fun refactoring ideas :-)

[fredvd]

the more-link stuff was added by us a few years for a customer project, but it seems to have been moved to the contentchooser.css in a previous clean up. The question is where the definitions should go. There's a generic cover.css and there's layout_edit.css which is hard coded in the layout view. more links are in list tiles on the compose screen, to stick with the current set up we should create a compose.css.

The different resources are annoying to override though: for css you can add higher specific selectors to your theme css, but the resource directory files cannot be overruled with z3c.jbot. When I had the need before i removed the cover resource from the resource registry with upgrade steps and registered my own version, but that becomes a maintenance mess as well. See #756

[hvelarde]

we're going to refactor all static resources to use webpack and after that we're going to completely refactor the content chooser with Vue.js.

[idgserpro]

@djowett #more-link-field it's present in more_link.pt which is only used by authenticated users. So contentchooser.css can still be displayed only to authenticated users, correct?