Enable new Profile manager

[rihards-simanovics]

in relation to #5741 and #805. We need a way to manage profiles in Firefox. There is now a new flag, browser.profiles.enabled, which enables a new modern profile manager. After having used it for a while, it appears to have all the existing (ala old) profile manager features plus a new UI with icons for each profile. It would be nice if that were enabled in Zen. One thing to note is that old-style profiles will not be ported and will still need to be accessed via the about: profiles URL.

[MulverineX]

I think it'd be really nice if you had the option to display shortcuts to your other profiles next to your Workspaces, also ideally you could create a new Profile with all the Settings of your default profile, just with fresh cookies, history, extensions, and bookmarks. (then after the fact if you want to change the theme or any other settings for that profile you can)

Restoring your Default profile's settings in a new profile is really annoying to do, as most of it you will always want for every profile.