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/read.go

@@ -0,0 +1,28 @@
+package storage
+
+import (
+	"encoding/json"
+	"errors"
+	"io"
+	"os"
+
+	"github.com/qdm12/gluetun/internal/models"
+)
+
+func readFromFile(filepath string) (servers models.AllServers, err error) {
+	file, err := os.Open(filepath)
+	if os.IsNotExist(err) {
+		return servers, nil
+	} else if err != nil {
+		return servers, err
+	}
+	decoder := json.NewDecoder(file)
+	if err := decoder.Decode(&servers); err != nil {
+		_ = file.Close()
+		if errors.Is(err, io.EOF) {
+			return servers, nil
+		}
+		return servers, err
+	}
+	return servers, file.Close()
+}