Implement the storage layer of the site metrics system

[GoogleCodeExporter]

We plan on introducing a metrics system to track user flows through the site, 
as detailed here:
https://docs.google.com/document/d/1AG1UeueSAH6XSyN9EYrwz9hJl-TAQPqJHLQakMxPaZs/
edit#heading=h.nvfzryp2d90n
This will be very valuable to us in identifying aspects of the site that need 
improvement, and trying different possibilities.

The first step is to implement a data model in which to store the metrics 
information after it has been compiled. This should basically be a table of the 
seventeen different data points in the "cron job" section of the document, 
crossed with the different iterations of oppia. Note that what constitutes an 
"iteration" (normally it will be a version) will be specified in an admin-page 
argument so it might be helpful to add that first.

Once this issues has been carried it out it will be possible to undertake 
further steps to implement metrics, namely:
 - A front-end to display them on the admin page.
 - A cron job framework that is able to periodically compile metrics.
 - Populating the cron job with the seventeen different desired metrics.

Note: if particular difficulty is encountered with compiling certain metrics 
then please let me know, as we have a degree of flexibility as to exactly what 
we measure.

Original issue reported on code.google.com by jacobdav...@gmail.com on 24 Mar 2015 at 1:17

[GoogleCodeExporter]

Assigning to Yuan for initial analysis/design.

Original comment by s...@seanlip.org on 2 May 2015 at 12:45

[GoogleCodeExporter]

Also, an overall suggestion: I think we should probably scope this down to 2 or 
3 representative metrics in the first pass, and implement a system that can 
handle these end-to-end, as well as easily generalize to include additional 
ones.

Original comment by s...@seanlip.org on 2 May 2015 at 12:59