bv assignment: adding task to fix assign philister users to bvs if they are not assigned, yet.

Call with `rake fix:bvs`.

Trello: https://trello.com/c/D1rIIleb/576-bv-19b,
https://trello.com/c/VCeC7ne5/586-fehlende-bv-zuordnungen

Wiki:
https://github.com/fiedl/wingolfsplattform/wiki/Zuordnungen-zu-Bezirksve
rbänden