Claim/unclaim packages for LTS work

Information is tracked in data/dla-needed.txt in the LTS security tracker.

See https://lts-team.pages.debian.net/wiki/Development.html for details.

Unclaiming a package with no CVEs left

If you claimed a package, and as a result of triaging there are no more open CVEs for it, instead of unclaiming you can completely remove the package from data/dla-needed.txt.