Theme resolving doesn't work in dev mode when having a servlet path.

[caalador]

When running an application in development mode with a servlet path part the theme url translation will not find any files from WebJars as the servlet will add the urlPattern in front of the import in toServletContextPath.

When running in production mode this won't cause problems as webJars are not used.

Tested with: alpha21

Steps to reproduce:

Run Skeleton starter flow with urlPattern "/app/*"

this will lead the import /./../bower_components/vaadin-button/theme/lumo/vaadin-button.html to become /app/./../bower_components/vaadin-button/theme/lumo/vaadin-button.html which won't resolve correctly in the webJarServer