add validation pattern for permission name

scm-webapp/src/main/java/sonia/scm/api/v2/ValidationConstraints.java

@@ -0,0 +1,12 @@
+package sonia.scm.api.v2;
+
+public class ValidationConstraints {
+
+  /**
+   * A user or group name should not start with the @ character
+   * and it not contains whitespaces
+   * the characters: . - _ are allowed
+   */
+  public static final String USER_GROUP_PATTERN = "^[^@][A-z0-9\\.\\-_]|([A-z0-9\\.\\-_]*[A-z0-9\\.\\-_])?$";
+
+}