Move common CSS styles out from the aplus-manual course into the a-plus-rst-tools theme or A+

[markkuriekkinen]

The CSS file _static/css/course.css contains some styles that should not be defined in the course but in the common tools like the theme inside a-plus-rst-tools, or sometimes possible even directly in the CSS of a-plus. For example, the aplus-manual course should not need to modify the admonition styles since that style should be provided to everyone as a default without requiring every course to copy the CSS into the course git repo.

This discussion started in #15.