Consider new file role: `hidden_but_downloadable`

[MrSerth]

Some teachers have wished for an additional file role, called something like hidden_but_downloadable (name up for discussion). The idea would be to have some files that are hidden in the web interface but can be retrieved when downloading the exercise (as a student). The idea would be that this file could contain special data only relevant on a learner's PC (and the integration with RStudio, Eclipse or another IDE).

Before implementing, let's discuss the use case once again and validate the necessity.