Add _admin_route option to all masquerade routes

Problem/Motivation

It would be nice if the masquerade routes are marked as admin routes. This will ensure that these routes will be rendered in the admin theme. Also when using https://www.drupal.org/project/admin_toolbar_messages, the masquerading messages will be displayed in the admin toolbar / navigation top bar.

Proposed resolution

Add the _admin_route option to all masquerade routes.

Remaining tasks

None

User interface changes

Masquerade routes will be rendered using admin theme.

API changes

None

Data model changes

None