Revert "Merge branch '21801-remigrate-epic-user-mentions' into 'master'"

Reverts !25259 (merged), which had failed post-deploy migrations on production due to timeout. !26777 (closed) was opened to revert this, but never got merged and then became out-of-date.

Reverted in 12-9-auto-deploy-20200311 as f47d0fe6, however there were conflicts when reverting this in master, which I've attempted to resolve here. We'll see how the specs behave!