Make "public" pastes meaningfully visible to logged-out users

Summary:
  - Introduce `shouldAllowPublic()`, indicating that logged-out users are OK in a controller if the install is configured to permit public policies.
  - Make Paste views and lists allow public users.
  - Make UI do sensible things with respect to disabling links, etc.
  - Improve behavior of "you need to login" with respect to policy exceptions and Ajax requests.

Test Plan: Looked at "public" paste, saw all unavailable UI disabled, clicked it, got appropraite prompts.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D3502

src/aphront/configuration/AphrontDefaultApplicationConfiguration.php

@@ -336,6 +336,18 @@ class AphrontDefaultApplicationConfiguration
     }
 
     if ($ex instanceof PhabricatorPolicyException) {
+
+      if (!$user->isLoggedIn()) {
+        // If the user isn't logged in, just give them a login form. This is
+        // probably a generally more useful response than a policy dialog that
+        // they have to click through to get a login form.
+        //
+        // Possibly we should add a header here like "you need to login to see
+        // the thing you are trying to look at".
+        $login_controller = new PhabricatorLoginController($request);
+        return $login_controller->processRequest();
+      }
+
       $content =
         '<div class="aphront-policy-exception">'.
           phutil_escape_html($ex->getMessage()).