Pluralize all the things!
Summary:
(Well, not all of them, but many of them - add pluralization to most strings that need it). See T16378
Also remove the `(s)` plural hack from a few places where it's logically impossible for there to only be one of the given thing, and fix a few random typos, while I'm at it.
Test Plan:
Run P60 and see fewer errors.
Read the code. Try to get some of the strings to show up in the interface but some of them seem to be unfindable.
Reviewers: O1 Blessed Committers, valerio.bozzolan, avivey
Reviewed By: O1 Blessed Committers, valerio.bozzolan, avivey
Subscribers: avivey, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26546