Maint: internal/storage rework

- No more global variables
- Inject merged servers to configuration package
- Fix #566: configuration parsing to use persisted servers.json
- Move server data files from `internal/constants` to `internal/storage`

internal/storage/servers.go

@@ -0,0 +1,7 @@
+package storage
+
+import "github.com/qdm12/gluetun/internal/models"
+
+func (s *Storage) GetServers() models.AllServers {
+	return s.mergedServers.GetCopy()
+}