Problem Set Update - Alternative winning lines improvements

[richard]

An important problem set update has been made. Lots of positions which had alternative winning lines have either been removed or had their alternatives explicitly recorded. If moves recorded as alternative winning lines are played, the user is told they have played a good move but asked to look for another one. No points are lost for choosing a recorded alternative. I'm hoping this change leads to a signficant improvement in perceived problem set quality, especially for higher rated players.

The threshold for being considered an active user has been dropped from an RD of 140 to an RD of 80. This should allow a wider range of users to appear in the top rated lists due to inactive users dropping out more quickly.  I had recently made a change which slowed down the rate at which RD increases over time, so this change also helps offset that slightly.

Richard.