feat: add link-based configuration emails (#725) (#754)

* feat: add link-based configuration emails (#725)

* fix tests

internal/domain/encoding.go

@@ -0,0 +1,29 @@
+package domain
+
+import (
+	"encoding/base64"
+	"strings"
+)
+
+// Base64UrlDecode decodes a base64 url encoded string.
+// In comparison to the standard base64 encoding, the url encoding uses - instead of + and _ instead of /
+// as well as . instead of =.
+func Base64UrlDecode(in string) string {
+	in = strings.ReplaceAll(in, "-", "=")
+	in = strings.ReplaceAll(in, "_", "/")
+	in = strings.ReplaceAll(in, ".", "+")
+
+	output, _ := base64.StdEncoding.DecodeString(in)
+	return string(output)
+}
+
+// Base64UrlEncode encodes the given input using the URL-safe base64 variant that the WireGuard Portal
+// API expects. In comparison to the standard base64 encoding, it uses . instead of +, _ instead of /
+// and - instead of =.
+func Base64UrlEncode(in string) string {
+	out := base64.StdEncoding.EncodeToString([]byte(in))
+	out = strings.ReplaceAll(out, "+", ".")
+	out = strings.ReplaceAll(out, "/", "_")
+	out = strings.ReplaceAll(out, "=", "-")
+	return out
+}