Why this change?
When a site's default locale is changed, Discobot's `UserProfile#bio_raw` is not
changed and we have gotten reports about this.
What does this change do?
This change adds a `DiscourseEvent.on(:site_setting_changed)` callback
which watches for changes to the `default_locale` site setting and
updates Discobot's `UserProfile#bio_raw` when it changes.
