Use a separate container for importing

[inodb]

We currently use a single container for both data importing. Let's split this up in web and data importing. There is a PR that set up the container for cbioportal-core: cBioPortal/cbioportal-core#16.

A few things are remaining.

Need to discuss:

• Remove inclusion of cbioportal-core in the final cbioportal Docker image during the Docker build process cBioPortal/cbioportal@master/docker/web-and-data/Dockerfile#L48. Or maybe remove that version of Dockerfile as a whole?
• The cbioportal-core name doesn't quite seem to cover what this repo does. Should it be called cbioportal-importer? Or cbioportal-data-import? Or something like that?

TODO:

• Update tests to use this new importer image
• Update cbioportal-docker-compose to use the new importer docker image
• Update documentation pages with how to use importer docker image directly