Support for cockpit 273+

[stanislavlevin]

cockpit 221 deprecated usage of its patternfly.css.
https://cockpit-project.org/blog/cockpit-221.html:

Developers: Importing “base1/patternfly.css” is deprecated
This pre-compiled stylesheet will be dropped in the future in favor of projects shipping their own CSS. This API is not maintainable, as Cockpit cannot offer a PatternFly 3 API forever, and PatternFly 4 also changes quickly enough that one style sheet for all projects is not robust enough.

The Cockpit plugins that are using only PatternFly 4 should follow the example from starter-kit on how to import PatternFly 4 stylesheets. g The Cockpit plugins which are still relying on PatternFly 3 should follow the migration from the deprecated API to the new PatternFly stylesheet import approach as implemented in this cockpit-podman commit.

cockpit 273 (cockpit-project/cockpit@e4b963b) dropped patternfly.css and cockpit.css.

fc-admin fails to load base1/cockpit.css.