renames usage state name for auth token

as noted, sessions might not be a good name for this metrics.
while devices would be a better name for users I think we should
align the name with the code as much as possible. The ui listing
all auth_tokens per user should probarbly say "devices" instead

pkg/models/stats.go

@@ -15,7 +15,7 @@ type SystemStats struct {
 	FolderPermissions     int64
 	Folders               int64
 	ProvisionedDashboards int64
-	Sessions              int64
+	AuthTokens            int64
 }
 
 type DataSourceStats struct {