feat(autocomplete): fix width in the header desktop + component improvements

[thecristen]

Summary of changes

Asana Ticket: Mostly no ticket, but final commit fixes Search bar visual bug

This is the base branch for #1805

• Styling changes to fix the visual bug
• For the algolia_autocomplete component:
  • Makes the placeholder attribute configurable (keep the current value as the default)
  • Changes how data-algolia attribute is populated slightly: using a nil value here will allow the attribute to the omitted. Makes the output a bit cleaner / easier to test.
  • Adds an attribute to indicate the desired event handler for the autocomplete's onStateChange property. I'm open to feedback on this because it's a little hacky-looking to me (it depends on a JS object with certain key names), but I wasn't sure of a better way to make onStateChange configurable from the Elixir templates.

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General checks

• Are the changes organized into self-contained commits with descriptive and well-formatted commit messages? This is a good practice that can facilitate easier reviews.
• Testing. Do the changes include relevant passing updates to tests? This includes updating screenshots. Preferably tests are run locally to verify that there are no test failures created by these changes, before opening a PR.
• Tech debt. Have you checked for tech debt you can address in the area you're working in? This can be a good time to address small issues, or create Asana tickets for larger issues.