Bug 1866802 - Move ASRouterAdmin tool to about:asrouter and its own component folder. r=pdahiya,Gijs,desktop-theme-reviewers,dao

This tries to maintain stylistic continuity, while also trying to decouple from
newtab as much as possible. This is a first foray, and future patches will
further this decoupling.

This also modifies about:asrouter to show an error message if the ASRouter devtools
pref is not set to true.

Differential Revision: https://phabricator.services.mozilla.com/D194811