first work on getting GPX out of the DB

This does a lot of changes already. What is definitely not working yet
are the tests, as they still try to put the data into the database - but
that should be easy to fix.

The convenience methods Track.{length,uphill,...} were a bit stupid to
fix, as our template code assumed that those attributes can just be
accessed. As a fix, I've introduced a new class that "re-introduces"
those and can lazily load them from the disk in case the cache does not
exist. This works pretty well, but is not too nice - we end up with a
lot of "proxy" properties.

Other than that, I'm positively surprised how well this has worked so
far, the upgrade scripts seem to be doing good and serving the file
straight from the disk seems to work nicely as well. What isn't tested
yet however is "edge cases", in case a data directory goes missing, ...

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