Add issue numbers / clean up all TODOs and FIXMEs in our codebase

[oswinso]

It's pretty triggering to see a TODO / FIXME that hasn't been touched for a good 10 years because nobody knows the context / forgets about the TODO. Let's add issue numbers to all relevant TODOs that are still in our codebase, delete the ones we don't know / aren't relevant anymore so that from now on at least the TODO is linked to an issue on github and people 10 years down the road know why the TODO exists and that its not forgotten.

AC: All FIXMEs converted to TODOs. All TODOs have issue numbers attached to them with the syntax: TODO(#issue_number): Brief description of the todo