fix user listing and search on non-US locale

This commit fixes two issues that appear on systems with non-US:

* Trying to show the list of users (as admin) fails when Postgre is complaining that it does not understand the date format "01-01-1970". I changed that to the ISO format, "1970-01-01".

* Search is broken and it does not find words that are clearly present. This also causes several tests in spec/components/search_spec.rb to fail. I fixed that by forcing the "english" config (stemmer), because discourse is using this config anyway on other places, e.g. when updating the search index.