Get rid of utils/lumo.ts helper/boilerplate

[Haprog]

Get rid of the utils/lumo.ts helper/boilerplate which is currently needed for being able to easily load Lumo styles into LitElement based views/components.

This depends on vaadin/vaadin-lumo-styles#90 so that the Lumo styles can be easily imported e.g. as a CSSResult object so that we could import those directly without needing to provide the lumo.ts helper in this project.

This has been split from issue #8 (comment)