Consider switching our database datatype for `discussion_id` in all tables

The following discussion from !4213 (merged) should be addressed:

• @abrandl started a discussion: (+2 comments)

  What format is discussion_id? An example from notes is 0000011070b9387d90b8ae48f5802e679eb12293. I was wondering if there's a more efficient way to store this (like we do for SHAs)? @mdelaossa @smcgivern