Replace custom HTMX tooling with django-htmx

[jeremystretch]

Proposed Changes

NetBox currently employs HTMX for dynamic rendering of certain UI views. This issue proposes replacing our related custom tooling (e.g. is_htmx()) with functionality provided by the django-htmx package.

Justification

Now that we've committed to using HTMX for enabling a more responsive UI, it makes sense to adopt a library dedicated to supporting HTMX operations and avoid reinventing the wheel.