Add Purge Threshold

Adds a purge threshold setting. This setting allows the user to
control at what filled percentage the purge activities are run.
This replaces the default, hard coded 95% point.

The purge threshold defaults at the original 95%. I set a minimum
of 20 and max of 99 because those values felt sensible but am open
to changing those based on feedback.

Note: The purge threshold is still active when the keep option is
set. I added a note for this but this still presents some risk
where users who change this while in Keep mode could have their
services shut down earlier than they expect.

Patch: Fix a couple of typos in initial changes and improve
formatting.

scripts/update_birdnet_snippets.sh

@@ -101,7 +101,7 @@ if ! grep -E '^COLOR_SCHEME=' /etc/birdnet/birdnet.conf &>/dev/null;then
 fi
 
 if ! grep -E '^PURGE_THRESHOLD=' /etc/birdnet/birdnet.conf &>/dev/null;then
-  echo "PURGE_THRESHOLD=\"95\"" >> /etc/birdnet/birdnet.conf
+  echo "PURGE_THRESHOLD=95" >> /etc/birdnet/birdnet.conf
 fi
 
 if ! grep -E '^MAX_FILES_SPECIES=' /etc/birdnet/birdnet.conf &>/dev/null;then