[chore/performance] Update media prune logic, add extra CLI command (#1474)

* start updating media prune stuff a wee bit

* continue prune / uncache work

* more tidying + consistency stuff

* add prune CLI command

* docs

* arg

testrig/config.go

@@ -34,7 +34,7 @@ func InitTestConfig() {
 }
 
 var testDefaults = config.Configuration{
-	LogLevel:        "info",
+	LogLevel:        "trace",
 	LogDbQueries:    true,
 	ApplicationName: "gotosocial",
 	LandingPageUser: "",