Test on Blacklight 9

[corylown]

Includes much of #1628
Drops Blacklight 8

[seanaery]

This is great - thanks for working through these changes! I see that the three test environments in CI are all now using BL9 and all have passing tests. I can get the app to run without error now using this branch. The UI will definitely need some subsequent work to catch up to all the BL9 changes :-)

[corylown]

@seanaery did you have any particular issues or changes in mind when you mentioned:

The UI will definitely need some subsequent work to catch up to all the BL9 changes

[seanaery]

@corylown I did -- mostly some obvious visible layout issues, but there's also probably some under-the-hood refactoring to do, like switching to using CSS custom properties (variables) instead of Sass variables. And now that BL9 fixes native advanced search, I want to ensure that a newly-generated arclight app's advanced search "just works." I'll aim to make some tickets today that include screenshots.