Limit rule to 255

To avoid database error

src/Wallabag/CoreBundle/Entity/TaggingRule.php

@@ -28,6 +28,7 @@ class TaggingRule
      * @var string
      *
      * @Assert\NotBlank()
+     * @Assert\Length(max=255)
      * @RulerZAssert\ValidRule(
      *  allowed_variables={"title", "url", "isArchived", "isStared", "content", "language", "mimetype", "readingTime", "domainName"},
      *  allowed_operators={">", "<", ">=", "<=", "=", "is", "!=", "and", "not", "or", "matches"}