Block spam messages on bug reports

I'm getting annoyed at all the spam messages coming through the bug report system.

Luckily, most spammers are quite stupid and won't be able to resist filling out the new hidden field "leave_empty_spamcatcher" in the bug report form. Boy will they regret it though, because doing so will make the bug submission fail and save us from their offers for cheap hotels and drugs.

Local tested and dev approved. (Tested locally and deployed to dev environment to verify behaviour)

P.S. From the reports it appears spammers use IE6 on Windows XP, they should upgrade their systems!