[Plan] Refactor Views to use HTMX instead of AJAX

Problem/Motivation

VIews makes extensive use of AJAX for the view building UI; Multiple components within the view designer are updated by AJAX calls, as well as displaying various pop-up forms, and updating the view preview. HTMX should allow the update logic to be simplified.

This is linked to #3548100: [PoC] Use HTMX for (some?) ajax views, but I think separate, as that only covers the UI of displaying a view, not the view builder UI.

Steps to reproduce

N/A

Proposed resolution

Review AJAX calls and review what's needed to replace with HTMX.

Remaining tasks

1. Make an inventory of AJAX calls within the Views UI.
2. Establish appropriate patterns for each type of AJAX request.
3. Agree a strategy to replace the AJAX calls.
4. Review test coverage and ensure adequate for changes.
5. Implement the changes and ensure tests pass.
6. Review and ensure community requirements met.
7. Ensure that key learnings are shared with the community.

User interface changes

While there is probably scope for improving the Views UI, I think this issue should just update to use HTMX. Other improvements could be tackled in follow-on issues.

Introduced terminology

API changes

Data model changes

Release notes snippet