New target: words studied. Today leaderboard and other fixes

CHANGE LOG - Features - New target: Added "words studied" as a target metric - Added "TODAY" as a mini leaderboard option - this is now set as default. NOTE: "Today" is based on the current day of the week. User scores are recorded in "Today" if they were on the same day, regardless of their time zone. - Fixes - Saving a target with 0 as the value will now disable it - Fixed order of targets - most commonly used targets are nearer the top of the page