Refactor SessionUtil, and Add dropdown menu to switch clientId

SessionUtil is renames AuthenticatedRegistrarAccessor, as it's used to access a registrar for an authenticated user.

It will now be injected with the AuthResult instead of receiving it in every function call, since there's only one "legal" AuthResult to use.

The AccessType names are changed from READ_ONLY/READ_WRITE to READ/UPDATE, as it was confusing that a user could have both READ_ONLY AND READ_WRITE access to the same registrar.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=216958306

java/google/registry/ui/soy/registrar/Console.soy

@@ -23,6 +23,8 @@
 {template .main}
   {@param xsrfToken: string} /** Security token. */
   {@param clientId: string} /** Registrar client identifier. */
+  {@param readWriteClientIds: list<string>}
+  {@param readOnlyClientIds: list<string>}
   {@param username: string} /** Arbitrary username to display. */
   {@param logoutUrl: string} /** Generated URL for logging out of Google. */
   {@param productName: string} /** Name to display for this software product. */
@@ -75,7 +77,33 @@
 
 /** Sidebar nav. Ids on each elt for testing only. */
 {template .navbar_ visibility="private"}
+  {@param clientId: string} /** Registrar client identifier. */
+  {@param readWriteClientIds: list<string>}
+  {@param readOnlyClientIds: list<string>}
+
   <div id="reg-nav" class="{css('kd-content-sidebar')}">
+    <form>
+      <select name="clientId"
+              id="select-client-id"
+              class="{css('kd-button')} {css('kd-button-submit')}"
+              onchange='this.form.submit()'>
+        <option value="">[auto select]</option>
+        {if length($readWriteClientIds) > 0}
+          <optgroup label="READ/WRITE">
+          {for $id in $readWriteClientIds}
+            <option value="{$id}" {if $id == $clientId}selected{/if}>{$id}</option>
+          {/for}
+          </optgroup>
+        {/if}
+        {if length($readOnlyClientIds) > 0}
+          <optgroup label="READ ONLY">
+          {for $id in $readOnlyClientIds}
+            <option value="{$id}" {if $id == $clientId}selected{/if}>{$id}</option>
+          {/for}
+          </optgroup>
+        {/if}
+      </select>
+    </form>
     <ul id="reg-navlist">
       <li>
         <a href="#">Home</a>
@@ -159,6 +187,12 @@
       <a href="{$logoutUrl}"
          class="{css('kd-button')} {css('kd-button-submit')}"
          tabindex="-1">Logout and switch to another account</a>
+      {if isNonnull($clientId)} // A clientId was given - but we don't have access to it
+        {sp}
+        <a href="?"
+           class="{css('kd-button')} {css('kd-button-submit')}"
+           tabindex="-1">Find registrar associated with your account</a>
+      {/if}
     </div>
   </div>
 {/template}