implement pluralization for server-side and js

Current status: Server side seems to support plurals, but only English pluralization rules (one vs. many) are available. Client side (JavaScript) has the same issue.

This patch implements server-side pluralization by including rules from https://github.com/svenfuchs/i18n/blob/master/test/test_data/locales/plurals.rb and instructing I18n to include pluralization rules. Client side pluralizations are assigned into i18n-js inside `app/assets/javascript/locales/xx.js`.