Reduce the size of the TODOs table

This issue is intended to house discussions on how we could reduce the size of the TODOs table.

Right now, we store very little data per TODO, but store all TODOs indefinitely so the table (on GitLab.com) is pretty large at just over 100GB.

Recently, there were limitations implemented on large tables which mean that we cannot add any new columns or indexes until we get under 100gb and 50gb respectively. We need to do something about this.

One idea we're exploring is the introduction of a data retention policy, but there are other ways we can do this. Lets discuss below and spin some ideas off into other issues.