[bug 889913] Track l10n coverage % for all locale/product combos.

* Created new model for these metrics
* Added cronjob to calculate and store values

I was torn between, (re)(ab)using the metrics model in the kpi app. In the end, I decided to create a new model closer to where it will be used and have the locale and product fields as columns. I could've built the locale and product into the metrics kind code using the kpi model, but then it would be awkward to query and we would have issues if a product slug changed, for example. I probably can be convinced to change my mind, but I still think this is the better option.

r?