This also rewrites the text to better explain what's happening. It's
likely further polish would be valuable, but that's true for the whole
"Troubleshooting" page.
This block of text was misplaced when we split the long
maintain-secure-update; article; we want it to be easy to find by
folks who are looking into error emails Zulip is sending.
Merges the "Upgrades" section from production/maintain-secure-upgrade.md
with production/modifying-zulip.md.
Contains significant textual changes by tabbott to read more clearly.
Mateusz Mandera
Tim Abbott
David Rosa