Updated for Wagtail 5.0

[katdom13]

Release notes: https://docs.wagtail.org/en/stable/releases/5.0.html

• Code cleanup
• Updated CHANGELOG

generic_chooser/templates/generic_chooser/_create_tab.html

• Updated longrunning button accordingly: https://docs.wagtail.org/en/stable/releases/5.0.html#progress-button-button-longrunning-now-relies-on-data-attributes

All tests run OK