Exclude fake source URLs from mappings CSV

Some document sources have fake old URLs which were added on import.
These URLs have paths containing 'fabricatedurl' or 'placeholderunique'.
Because these URLs never existed, they should not be included as old URLs
in the mappings CSV consumed by Transition.

It looks as though the existing URLs of this type are on document
sources, but check attachment source URLs as well for safety.
Merge pull request #1222 from alphagov/exclude_fake_source_urls_from_mappings_csv

Exclude fake source URLs from mappings CSV