Update data models, layout, documentation

[anorthall]

• Remove the concept of approved incidents.
• Added list of injury types to be implemented in the future.
• Remove references to approving incidents and hide the approval process.
• Hide the 'Primary/Secondary Cause' fields.
• Change 'SPAR' to 'Self rescue'.
• Remove django-import-export plugin
• Tidy up settings.py and remove unused code.
• Redesign home page.
• Update all editor documentation.
• Redesign references section on Incident pages.
• Remove dependency on Publication being present for each Incident, in preparation for accepting non-ACA incidents.
• Remove some wording that specifically mentioned 'ACA reports' where it referred to 'incidents', in preparation for accepting non-ACA incidents.
• Categorise Country/US States using choices fields.
• Change 'Category' field on 'Incident' to be only Cave, Mine, Cave Diving, Other.
• Renamed created and updated fields to end with _at.
• Change the 'incident add text' page to pull data from S3.
• Add a footnote to each incident that was processed by AI.
• Add some basic database constraints to Incident.
• Remove checks/crosses from Incident page titles (previously indicating complete/incomplete).
• Minor changes to styling of headers in dark mode.
• Update requirements.