Allow exporting the ranklist in a CSV format

[valo]

May we we should allow this only to coaches as it doesn't make much sense for the regular users to be able to do that.

[SValkanov]

The idea is to use something like 'whenever' and generate the csv each day, or it will be generated on each run for export?

[valo]

The ranklist can be cached in the rails cache for certain amount of time. There is a memcached service running on production, so it should work pretty well. Something like this: http://guides.rubyonrails.org/caching_with_rails.html#low-level-caching

We should evaluate how much time and storage it will take to compile it. It may turn out it is very expensive to generate it, in which case we can have a background job, which generates it and refreshes it in the cache. It will need some benchmarking and analysis.