Remove constraints on minimal size of posts.

NOT_EMPTY is the only way to forbid posting nothingness.
Slightly reduce the size of core.js.
Display exception contexts on error pages.

I love exceptions.
Slightly redesign database layout.

Backward compatibility is, obviously, lost.

 * RP_THREAD is now replaced by RP_TREES, which also stores Tree object in addition to Threads. This allows for faster dereferencing.
 * Root posts of threads have their `thread` field set to their own id, not 0.

Known bug: whenever a board overflows, some threads appear to have been removed but, in fact, still remain in the database.