Derive RDAP link paths from the received query

The former method -- a config string -- was cumbersome, as each Nomulus system would have to configure the link base to its own URL.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=173567021

javatests/google/registry/rdap/RdapHelpActionTest.java

@@ -48,9 +48,9 @@ public class RdapHelpActionTest {
 
     action = new RdapHelpAction();
     action.clock = clock;
+    action.fullServletPath = "https://example.tld/rdap";
     action.response = response;
     action.rdapJsonFormatter = RdapTestHelper.getTestRdapJsonFormatter();
-    action.rdapLinkBase = "https://example.tld/rdap/";
     action.rdapWhoisServer = null;
   }