problems.utils topological_sort returns also level list

Pequeña modificación para que además del orden topológico devuelve la
lista con los niveles de cada nodo/problema.
problems. user detail: differenciate problems locked/unlocked

user_detail view: added dbqueries function to get next problems to
solve. passed list of identifiers of problems unlocked (cansolve) and
locked (nextsolve)

user_detail template: modified not solved problem list so unlocked
problems have a link and locked ones don't. Also for CSS they have
different <li> identifiers: cansolve, cantsolve