Add missing htmx:trigger event on load triggers

[alexpetros]

Description

The htmx:trigger event wasn't firing when the trigger was a load.

Testing

I added a new test for this scenario, verified that it passes for the fix, and that it doesn't pass without the fix.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded