Style a few more 'errormsg's in utils.php
Sanitize answers, titles, and other messages more uniformly

* Titles of puzzles, rounds, answers, and failing MySQL queries are run
  through htmlspecialchars; descriptions are purified with HTMLPurifier.
  All of this is done before insertion into the database, for
  consistency. (This may not be the best thing to do here --- now, the
  answer or guess ">.<" will be considered equal to "gtlt". This can be
  discussed further.)
Reformat puzzle-testing page

* swap some h2, h3, h4s around
* make the list of attempted answers <ul> instead of a table
* make the correct/incorrect answer messages stand out more
* Puzzletron is smart enough to know whether there are any correct
  answers registered, so have it tell the testsolver
* an &mdash;
Fix two more &mdash;es for no good reason
Highlight "new" puzzles more brightly / intuitively (?)