Code monitors: use match count, not result count in slack notification

Created by: camdencheek

Previously, we used result count in the title, but match count in the "n more matches" trailer. This is confusing because you might get a message that says "3 new matches" but then a trailer that says "5 more matches", which doesn't make any sense. This updates it to use match counts instead of result counts.

See old example:

Test plan

Updated tests demonstrate that this works as intended.