Solution scoring refactor; yes/no rating

Unintuitive 1-5 scoring has been replaced with correct/incorrect
scoring, where a solution is accepted if at least 60% of the votes are
positive.
Special design has been created for yes/no scoring, which for tehnical
reasons does not use AJAX.
Additionally, Solution status dictionary has been replaced with
constants (SolutionStatus and SolutionDetailedStatus class).

New images have been added, and a list of image sources has been
created.
Other minor changes and possibly bug fixes.