for MPP-3755: use Thread to send event data to GA async

[groovecoder]

This PR is for #MPP-3755.

How to test:

1. Run pytest
2. Inspect the Relay extension to capture a network request to /metrics-event endpoint
3. Use the "Copy as cURL" command to copy it
4. Paste the curl command to your command-line
5. Change the https://relay.firefox.com to http://127.0.0.1:8000
   • It should work!

Checklist:

• [x] l10n changes have been submitted to the l10n repository, if any.
• I've added a unit test to test for potential regressions of this bug.
• [x] I've added or updated relevant docs in the docs/ directory.
• [x] All UI revisions follow the coding standards, and use Protocol tokens where applicable (see /frontend/src/styles/tokens.scss).
• Commits in this PR are minimal and have descriptive commit messages.

[jwhitlock]

I verified this locally with curl -v http://127.0.0.1:8000/metrics-event --json '{"ga_uuid": "test"}'. The view returns, and then the thread runs and sends the message.

It did not run in pytest, because it doesn't have a chance while all the tests are running, and daemon threads are terminated even if not completed. I considered some options, and decided to mock the thread entirely and call the function on thread.start. This allows normal test assertions.

[jwhitlock]

Thanks @groovecoder. Feel free to review my changes and merge when ready.

[groovecoder]

Great, thanks for the extra testing!