Fix accordions in CiviCRM 5.69

Fixes issue #10 (closed) cc @Detlev @jon

It also removes the use of custom FontAwesome icons for the collapse/uncollapse icons. It's a small visual difference, but we aim to slowly reduce the amount of custom CSS.

In CiviCRM 5.69 before the fix:

With this fix:

Comparison with CiviCRM 5.65:

I also sent a PR upstream to fix the alignment of the text of the two accordion headers:

https://github.com/civicrm/civicrm-core/pull/29038

Needs a bit more testing to make sure it does not break other parts of CiviCRM.