Using the main branch of the profiler should be easy

To reduce the likelihood of deploying annoying regressions in production, it would be nice to have a few frequent users of the profiler use the main branch instead of the production branch.

Currently it's possible to change a pref in about:config to change the capture url, but that's not very helpful in this case because people are usually capturing profiles in instances of Firefox which isn't the one where they analyse profiles.

It would be great to have a way to have profiler.firefox.com redirect to another domain if the user opted in. This would like let the other domain be used for the case of clicking a share.firefox.dev link.

A possible way for the user to opt-in would be for the user to type something in the web console. Eg. `experimental.useUndeployedBranch()` or something similar, that would set a cookie or something in local storage, that would trigger a redirection when profiler.firefox.com is loaded.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-317)