Add captcha to member signup page

[padski]

Continuous spam signup is happening no doubt scripted

[zymurgic]

database check constraint added, signup page re-instated for now, will keep an eye on it until someone has time to actually add a captcha

[zymurgic]

Mark Steward has implemented the same sort of text-captcha as on the wiki. Looking good so far. The current signup spammer is still being kept at bay by the teergrube I put in the code, but if they change their modus operandi then it's likely to keep them at bay more.
Still monitoring. I wonder how long it will be until the spam scammer notices that nothing is actually happening.