Clean up registrar console login flow

Replaced the plethora of inter winding access functions and inputs in SessionUtils with just 2 functions, that both accept the same type for the user (AuthResult):

guessRegistrarForUser: given an AuthResult, finds a registrar that they have access to. If none is found - a ForbiddenException is thrown.

getRegistrarForUser[Cached]: (maybe should be called getRegistrarOnBehalfOfUser?) given an AuthResult and a clientId, loads and returns the registrar ONLY IF the user has access to it. Otherwise throws a ForbiddenException.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=214630657

java/google/registry/request/auth/AuthResult.java

@@ -36,6 +36,12 @@ public abstract class AuthResult {
     return authLevel() != AuthLevel.NONE;
   }
 
+  public String userIdForLogging() {
+    return userAuthInfo()
+        .map(userAuthInfo -> userAuthInfo.user().getUserId())
+        .orElse("<logged-out user>");
+  }
+
   public static AuthResult create(AuthLevel authLevel) {
     return new AutoValue_AuthResult(authLevel, Optional.empty());
   }