make it host a classroom - define tracks in a course

[parmentelat]

it would be rather simple to use this in the classroom

• teachers can define one or several viewpoint (structured subsets of notebooks)
  otherwise derived from the filesystem structure
• accounts are created out-of-band, prior to the class, using django tools
• authenticated users can browse the course contents and open notebooks without the need for an edx instance

there currently is a beginning of something along these lines in branch devel

[parmentelat]

this has been partly addressed in #69 and #70

another aspect of this feature is related tothe notion of tracks
a track is a point of view on a notebooks repo; it allows to organize a subset of the available notebooks into sections

so far we've used tracks in various contexts, in order to:

• offer an english track or a french track - see e.g. https://github.com/parmentelat/flotbioinfo/blob/master/nbhosting/tracks.py
• propose two complementary tracks, a course could have one common track and one options track - see e.g. https://github.com/flotpython/slides/blob/master/nbhosting/tracks.py
• propose a full track or exercises-only track - see e.g. https://github.com/flotpython/course/blob/master/nbhosting/tracks.py

as illustrated in these examples, each course can define its own tracks by writing a tracks.py module in its nbhosting subdirectory.

considering this closed as of 75ca167 aka nbhosting-0.9.0