Include localized khan exercises into language packs

Fixes #1222, dependent on #1286 ; proper diff can be found here: https://github.com/bcipolli/ka-lite/compare/1164...1222?expand=1

Includes the following changes:
* Scrape exercises from `update_language_packs`
* Refactoring of the metadata computations in `update_language_packs`
* Properly update language pack versions based on # exercises, # known videos
* Move downloaded exercises to proper location on the distributed server.

Testing:
* Tested thorougly from the central server side.  Note: this will not do the right thing for english.
* Tested just a bit from the distributed server side