Implement YAML redirects mechanism
Keep track of the Pages redirects in a YAML file and use a Raketask to populate _redirects.
This helps us keep any metadata we want that are irrelevant to Pages redirects, like the removal date.
I used the last known state with the dates to create a CSV file, then parsed that to create the YAML file. I still need to check with the updated redirects file to see if I missed something.
Closes #929 (closed)