404 problem report form improvements

This PR improves the problem report form on the 404 pages:

- the "what went wrong" field is no longer set by the user (1 week's worth of production data showed that users tended to either leave this field blank, state the obvious such as "the link is broken", or plain abuse)
- adjust the copy of the "what were you doing" field
- referrer is now captured
- "javascript is enabled or not" is now captured