Unicode username and search improvements

I will improve Unicode/i18n for Discourse, especially a Unicode username feature. This includes a new gem to leverage on battle-tested ICU library and a so called \emph{i18n backend}. The Unicode/i18n problem has already being handled by Rails but there are also limitation without improvement over years. Leveraging on Rails is not enough.

I'll also improve the CJK search. That includes a set of plugins for PostgreSQL so that the index quality can be further improved. Along with the refactors with search module, Discourse search for CJK forum should be improved.