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.

scripts/install_config.sh

@@ -136,6 +136,10 @@ COLOR_SCHEME="light"
 
 FULL_DISK=purge
 
+## PURGE_THRESHOLD can be set to configure at what disk full percentage the
+## purge operations are triggered.
+PURGE_THRESHOLD=95
+
 ## Maximum amount of files to keep for a given specie (0 = keep all)
 ## Files from the last 7 days, and files protected from purge, are not taken into
 ## account in this number