Add post_migrate signal handler to load custom SQL

This handler can be used to load SQL stored in files ending with .sql stored
inside the sql directory of the catmaid application folder.
Add basic PL/pgSQL function to create intersection table

This script currently only creates intersections for treenodes that are on
individual slices of a XY aligned stack.
Intersection tables: make DB name a parameter
Intersection tables: handle all three orientations in PL/pgSQL function
Intersection tables: remove debug output