Migrate from travis-ci.org to travis-ci.com

[foolip]

We now have the ability to migrate individual repos in the web-platform-tests org from travis-ci.org to travis-ci.com, and we have already migrated wpt.fyi as a trial. Old links and build history are preserved.

There isn't a strong reason to migrate now, but we eventually have to. One upside is that we get the new Checks API integration which is also what Azure Pipelines uses. The Travis integration isn't very fancy, but it's a small improvement. Example from wpt.fyi:
https://github.com/web-platform-tests/wpt.fyi/pull/910/checks?check_run_id=40636708

@web-platform-tests/admins I would like to do this migration org-wide. If I do not hear concerns within a week I'll go ahead.

[Hexcles]

Just a reminder that you'd need to change the "required checks" for protected branches after migration.

[foolip]

Yep. There are also details about cron jobs that I'll take care of.

[foolip]

I will do this now, one day earlier than planned, to give a few more days to clean up the mess if I break stuff.

[foolip]

The most recent PR is now #14600, any PRs created after then but before the transition is complete need to be inspected so they're not blocked on checks that will never run.

[foolip]

The app is now installed org-wide instead of just for wpt.fyi. Following the documentation in https://docs.travis-ci.com/user/open-source-repository-migration/ to migrate each individual repo.

[foolip]

#13352 has finished running on travis-ci.com, and I sent #14603 to test the full set of tests.

I've changed the required check from "continuous-integration/travis-ci" to "Travis CI - Pull Request", but unfortunately that makes it show up as required on all existing PRs as well. #14600 is an example of this, now in the state "Required statuses must pass before merging". I'll see if there's a way to trigger the runs without closing and reopening all PRs.

[foolip]

For master, https://github.com/web-platform-tests/wpt/runs/42781561 was the first job to run after the change, followed by https://github.com/web-platform-tests/wpt/runs/42784876. The worked as intended:
https://github.com/web-platform-tests/wpt/releases/tag/merge_pr_14570
https://github.com/web-platform-tests/wpt/releases/tag/merge_pr_13352

[foolip]

https://github.com/web-platform-tests/wpt/pull/14603/checks?check_run_id=42779464 shows what the new checks will look like. Closing that test PR now.

[foolip]

There doesn't appear to be anything in https://octokit.github.io/rest.js/#api-Checks to trigger just Travis checks for a PR. Still looking for a way to resolve #14499 (comment).

[foolip]

Per https://travis-ci.community/t/trigger-build-on-pr-via-api/825/2 it doesn't look like there's a way with the Travis API either.