Tighten route incoming_path validation

Reject paths with consecutive slashes, and with trailing slashes.  This
will avoid creating routes that are unique, but not semantically unique.
Cleanup removals from seeds.

These have been run, so can go now
Merge pull request #21 from alphagov/tighten_path_validation

Tighten route incoming_path validation
Merge pull request #22 from alphagov/cleanup

Cleanup removals from seeds.