Add _admin_route for field manager and admin/content

This is one approach, not sure whether it's the right one.
Still need admin/content though

AdminRouteSubscriber already sets _admin_route to TRUE on all admin/* paths, it just needs reordering so it runs after Field UI's and Views' own route subscribers.

Not sure about the -200, I picked it arbitrarily as Field UI's subscriber uses -100.

It seems fine to have that low number as long we have some little bit of documentation to actually know why it is done in that particular way.

Yes, this is more sane indeed. Both are now in the admin interface.

Hmm don't think so, quickly tested it and it was ok, maybe someone else should reverify.

AdminRouteSubscriber runs after \Drupal\views\EventSubscriber\RouteSubscriber so Views routes such as /admin/content are included.

We might want a separate Views page display option for "Use admin theme" but that is outside the scope of this issue.

Added a @todo pointing to #2158571: Routes added in RouteSubscribers cannot be altered, this shouldn't be necessary if we have separate add/alter stages. Attaching interdiff.

Committed/pushed to 8.x, thanks!