Improve information related to files shared across multiple courses

[danmarsden]

If the same file is used within multiple courses it will be counted in each course. It would be good if we could add information to each course about files that may be shared across other courses. For example a 150MB course could have 30MB shared with other courses so it would be nice for the disk usage column for the course to reflect this - maybe like:
150MB (30MB Shared)

[danmarsden]

I've just added a patch to add a new Shared usage column to the report - I've tried to make it cross-db compatible by using the helper functions and have tested this on Postgres and it works ok - there might be a more efficient way to do it though so if anyone has a look at the query (and can understand it) feel free to make any suggestions to improve it!

[danmarsden]

hmm - looks like the test I wrote for it is failing - must be something wrong with the query still - I'll look closer.

[danmarsden]

It's still not quite right - I need a way to backtrack the context path and find the course context path related to this individual file so that we can exclude duplicates from the course itself. I have a few ideas on this but for another day....