This adds support for using PGroonga to back the Zulip full-text
search feature. Because built-in PostgreSQL full text search doesn't
support languages that don't put space between terms such as Japanese,
Chinese and so on. PGroonga supports all languages including Japanese
and Chinese.
Developers will need to re-provision when rebasing past this patch for
the tests to pass, since provision is what installs the PGroonga
package and extension.
PGroonga is enabled by default in development but not in production;
the hope is that after the PGroonga support is tested further, we can
enable it by default.
Fixes#615.
[docs and tests tweaked by tabbott]
Kouhei Sutou