Move FACTOR_ constants to FactorGrantedAuthority

[rwinch]

Previously GrantedAuthorities had an implicit package tangle because it
was located in ~.core and FactorGrantedAuthority is in ~.core.authority
and FactorGrantedAuthority.authority property was implicitly expected
to be constants found in GrantedAuthorities.

This commit moves the constants to the FactorGrantedAuthority which
resolves this tangle. It wasn't initially done because
FactorGrantedAuthority did not exist at that time.