Move UrlRedirectSink into importable library

- The `UrlRedirect` class is renamed to `ServletUrlRedirect`.
- Abstract class `UrlRedirectSink` is defined that can be imported and
used to customise CWE-601 via Customizations.qll

java/ql/src/semmle/code/java/security/UrlRedirect.qll

@@ -0,0 +1,5 @@
+import java
+import semmle.code.java.dataflow.DataFlow
+
+/** A URL redirection sink */
+abstract class UrlRedirectSink extends DataFlow::ExprNode { }