Properly include OpenAPI client into project

[snkaupe]

The simplest way to use the Angular client is to deploy it into the src directory using a Git submodule. However, a cleaner way is to still use a Git submodule, but have it in a different folder (e.g. external) and to build a NPM package that is then included in the Angular project.

This is the way it is currently done. However, this requires a few changes to the OpenAPI client (which are not yet committed) as well as manual build steps that are currently neither automated, nor documented. To simplify further development, it might be useful to switch to the "lesser" inclusion method until such a time that the necessary resources to properly use an external NPM package are available.

[snkaupe]

I've done as suggested and moved the Git submodule for the OpenAPI-generated Angular client into the src tree under src/external and fixed all resulting errors as part of my PR #20. The issue will remain open, as moving it back into a proper folder for external dependencies would be preferable. But, as noted before, this would require changes to the OpenAPI repository as well, so we'll move it into the uncertain future, given that everything works.