Make the templates/category lists on edit page collapsible

This is a total rewrite of Vector extension's functionality, to fix
various accessibility and internationalization issues with the
original one. Both the JavaScript and the CSS have been redone from
scratch.

Still, the look&feel is almost the same as in the original version,
save for the mentioned messaging changes and minor UI differences (the
toggles are no longer <a> elements). The same cookie names have been
used to avoid any issues when migrating.

Used standard arrow icons from the mediawiki.icon module.

Bug: 43689
Change-Id: I91a3704cb09b9e17dd8baa516ab6f8ee441b7467