Made some classes final.

Summary:
Currently, the linter raises `XHP29` warnings for these files because they are not abstract or final.

I guess there are two possibly solutions, either making the classes final or marking them as `@concrete-extensible`. Given that there are no subclasses of these classes in the `phabricator`, `arcanist` and `libphutil` repositories... I opted to declare the classes as final.

Test Plan:
The following linter warnings are gone:

```
>>> Lint for src/aphront/configuration/AphrontDefaultApplicationConfiguration.php:

   Warning  (XHP29) Class Not abstract Or final
    This class is neither 'final' nor 'abstract', and does not have a
    docblock marking it '@concrete-extensible'.

               3 /**
               4  * @group aphront
               5  */
    >>>        6 class AphrontDefaultApplicationConfiguration
               7   extends AphrontApplicationConfiguration {
               8
               9   public function __construct() {

>>> Lint for src/applications/differential/mail/DifferentialReplyHandler.php:

   Warning  (XHP29) Class Not abstract Or final
    This class is neither 'final' nor 'abstract', and does not have a
    docblock marking it '@concrete-extensible'.

               1 <?php
               2
    >>>        3 class DifferentialReplyHandler extends PhabricatorMailReplyHandler {
               4
               5   private $receivedMail;
               6

```

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8347

src/aphront/configuration/AphrontDefaultApplicationConfiguration.php

@@ -1,6 +1,9 @@
 <?php
 
 /**
+ * NOTE: Do not extend this!
+ *
+ * @concrete-extensible
  * @group aphront
  */
 class AphrontDefaultApplicationConfiguration

src/applications/differential/mail/DifferentialReplyHandler.php

@@ -1,5 +1,10 @@
 <?php
 
+/**
+ * NOTE: Do not extend this!
+ *
+ * @concrete-extensible
+ */
 class DifferentialReplyHandler extends PhabricatorMailReplyHandler {
 
   private $receivedMail;