Follow any redirects present within the same file

See https://www.pivotaltracker.com/story/show/43492205 for rationale.
Add document_mappings.csv to gitignore

This file is present whenever we run the munge, but should not be checked in.
Handle circular redirects
Ensure we follow redirects more than one step
Improve efficiency of mapping generation

Do it only once as opposed to every row
Rerun DECC munge to test redirect following

The changed redirects are due to the previously targeted decc.gov.uk links actually being 410s in the same file.