Add user language preference setting

Add i18n fallback: untranslated strings render as English
Add language preference column to user table

Logic: use the browser header to determine which language to display for logged out users,
or for logged in users who haven't specified a language preference.

Modify user setting form to de-ajaxify it.
Merge pull request #666 from loomio/feature/language-setting

Add user language preference setting