NEW Allow specifying priority for translations

Priority for translations was hardcoded, and hardcoded the project name as "mysite".

This takes the order from a configuration property "module_prority". You can
use standard config fragment before and after rules to make a module less or
more important than anything else, with these tweaks:

 - Unless it has it's order explicitly defined, the "project" module (normally mysite)
   will be considered highest priority

 - There is an "other_modules" value in the order list which will be replaced by
   all the modules (except the project module) that don't have their order
   explicitly defined.
Added docs for i81n.moduole_priority.