Change label used to keep issues from being marked as stale to keepalive (#965)

The label used previously, important, implied precedence rather than
just a desire to keep an issue alive.

.github/stale.yml

@@ -6,7 +6,7 @@ daysUntilClose: 7
 
 # Labels that prevent issues from being marked as stale
 exemptLabels:
-  - important
+  - keepalive
 
 # Label to use to identify a stale issue
 staleLabel: stale