Check advisories

This MR adds a script to check if advisories are missing from the website. It parses data/*/list and looks for the relevant files in the webwml repository (whose path needs to be provided as an argument), e.g.:

$ bin/check-advisories --website-path ../../webwml
ERROR: .data or .wml file missing for DSA 4342-1 (reserved by Michael Gilbert)
ERROR: .data or .wml file missing for DSA 3156-1 (reserved by Luciano Bello)
ERROR: .data or .wml file missing for DSA 3043-1 (reserved by Moritz Muehlenhoff)
ERROR: .data or .wml file missing for DSA 2360-1 (reserved by Michael Gilbert)
ERROR: .data or .wml file missing for DSA 1975-1 (reserved by Stefan Fritsch)

$ bin/check-advisories --mode DLA --website-path ../../webwml
ERROR: .data or .wml file missing for DLA 2126-1 (reserved by Thorsten Alteholz)
ERROR: .data or .wml file missing for DLA 2125-1 (reserved by Thorsten Alteholz)
ERROR: .data or .wml file missing for DLA 2124-1 (reserved by Thorsten Alteholz)
ERROR: .data or .wml file missing for DLA 2114-1 (reserved by Ben Hutchings)
ERROR: .data or .wml file missing for DLA 2043-2 (reserved by Thorsten Alteholz)
ERROR: .data or .wml file missing for DLA 2031-1 (reserved by Hugo Lefeuvre)
ERROR: .data or .wml file missing for DLA 2000-1 (reserved by Hugo Lefeuvre)
ERROR: .data or .wml file missing for DLA 1714-2 (reserved by Hugo Lefeuvre)
ERROR: .data or .wml file missing for DLA 1713-2 (reserved by Hugo Lefeuvre)
ERROR: .data or .wml file missing for DLA 1953-2 (reserved by Hugo Lefeuvre)

The original script is from Antoine Beaupré from webmaster-team/cron!1 (closed) (which I will close when this is merged). I have gone through the DSA results and fixed a few entries in data/DSA/list which had wrong dates and caused the script to (rightfully) miss those files, and the security-tracker website to have broken links.