Refresh the site after updating certain site settings

Some site settings require a refresh on the client. Also, sometimes server configuration changes require a refresh. There is already a mechanism for this when Discourse updates. This PR provides a mechanism to do that in other situations.

### Breakdown
Method added to `discourse.rb`, `request_refresh!` - will publish 'clobber' to the `asset-version` channel, which will never be a valid SHA, causing the client to refresh on next navigation.

The warning about making people refresh after changing `allow_uncategorized_topics` is removed.

Updating certain site settings will call `Discourse.request_refresh!`. These are `allow_uncategorized_topics`, `default_locale`, and `authorized_extensions`.

Method added to `script/discourse` named `request_refresh`, which calls `Discourse.request_refresh!`.