Remove the "with_default"-style traits/factories

These were there as a vague stab at improving the performance of tests
at the expense of added complexity. What use is a site without at least one
host in it, though? Likewise a mapping that's not for a site? Remove these,
remove noise. Also, inexplicably, the specs get faster by about 6s. Win-win-win.
Trait consistency, this time for mappings
Remove duplicate factory mapping_410
Factory whitespace consistency
Merge pull request #112 from alphagov/factory-fix-pt2

Factory fix pt2