This closes#10146.
Because of cibernox/ember-power-select#1203, which documents
the current impossibility of attaching test selectors to a
PowerSelect invocation, this uses test selectors on parent
containers instead, occasionally adding wrappers when needed.
I chose to leave the existing test selectors in the hopes that
we can return to using them eventually, but I could easily
remove them if it seems like extra noise now.
Presumably for the same reason, @class no longer works, so
this adjusts the scoping of global search CSS to preserve the style
of the search control.
I also included an update to the latest version of
ember-test-selectors, since we were far behind and I tried
that before finding the aforelinked issue.
Finally, this replaces ember-cli-uglify with ember-cli-terser to address
production build failures as described at ember-cli/ember-cli#9290.
