This skeleton loader page is used in a few places where we're in an intermediate state and want to show the user something without having enough information about what to show them.

For example, when first opening the Migration Wizard, we'll want to show this state while the MigrationWizardParent goes off and scans the disk for available browsers and profiles (which, on slower spinny disks, might take on the order of hundreds of milliseconds):

This is the specification: https://www.figma.com/file/PcMkM0ZaSSy3zmS3CYGWNj/Handover---students?node-id=846%3A10374&t=Fx3JSSDXIvKSrzMu-4

Up until now, we've used the connectedCallback to initialize the MigrationWizard. That's
been fine, except that it requires us to remove and then re-add the element to the DOM
if we want to "reset" it and start over.

This patch adds a method "requestState" to the MigrationWizard that kicks off the
initialization. Embedders can set the auto-request-state attribute on the
element if they're happy to just use the connectedCallback.

Finally, this adds an intrinsic width to the entire MigrationWizard element to reduce
flicker when transitioning between states.

Depends on D171741

https://hg.mozilla.org/mozilla-central/rev/6b6dfc4ac693
https://hg.mozilla.org/mozilla-central/rev/9339270f6109