Re-arrange JS files name and location.

This PR contains breaking changes - I've rearranged the JS files to solve a few problems.

Fixes:
 * shims.js is correctly loaded when asset pipeline is not used
 * i18n.js replaces i18n/translations.js as the main loader when asset pipeline is enabled. As a result, export no longer overwrites the loader (export file location is unchanged)
 * Re-enable detection of assets pipeline during export (from 2.x) and exports to correct location at app/assets/javascripts/i18n when asset pipeline is enabled

Changes:
 * assets pipeline detection at js.rb
 * move i18n.js to i18n/i18n.js
 * move i18n/tranlasation.js (the loader) to i18n.js
 * rename filtered.js.erb back to translation.js.erb

These are more or less incompatibilities introduced when we merged rewrite over. I understand that this is another breaking change but hopefully this will make everything more straight-forward going forward.