Update about:welcome simplified full page with ability to change content via experiments (e.g. the title text) without landing code (i.e. normandy integration)

See https://bugzilla.mozilla.org/show_bug.cgi?id=1616370

Per @dveditz@mozilla.com from Security:

"I've gone over the review request and taken a look at the code. I'm happy to see that about:welcome itself and the remote control experiment mechanism follows the well-tested patterns used in other Messaging System projects like about:newtab.

I don't have any security concerns about the code design itself. As noted in your review request, changing the data on the Remote Settings server changes content in the client. It's not "hacking", but like most of our other Remote Settings, unauthorized changes would result in reputational harm and brand damage. I know from other projects that Remote Settings changes typically require two people, analogous to the review requirement for Firefox code commits. I don't know enough about how Remote Settings are managed to know if that's a blanket requirement or if each project has to set that up, but it should be in force here."

Considering all dependent bugs are resolved and the feature is successfully rolled out in 76, closing tracking bug as resolved