external_format_text should be safe to call from web or webservice

Or anywhere really.

The problem is that we want to write re-usable renderables - that can export their data for returning to a web-service - or rendering in the page.

This means that they either:
a) have to know if they are being called for webservice and call the correct format_text/external_format_text function
b) always should call external_format_text and it should do the correct thing.

B is sensible - A is not.

The problem with B at the moment - is that external_format_text by default does not apply any filters (it should match the default for format_text).

Another problem is that we do not have a matching function external_format_string for names etc.