Auto-subscription: little less verbose

Summary:
When the user "Mrs. Kitten" adds or remove "Mrs. Kitten" as Subscriber,
usually these messages were generated:

    Mrs. Kitten added a subscriber: Mrs. Kitten.
    Mrs. Kitten removed a subscriber: Mrs. Kitten.

This was a bit like the Spiderman meme self-pointing Spiderman.

After this change, these beautiful messages are generated instead:

    Mrs. Kitten subscribed.
    Mrs. Kitten unsubscribed.

| Before    | After     |
| {F286215} | {F286216} |

Closes T15347

Test Plan:
- subscribe on something
- unsubscribe from something
- all other cases remain as-is

Reviewers: O1 Blessed Committers, bfs, speck

Reviewed By: O1 Blessed Committers, bfs, speck

Subscribers: bfs, speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15347

Differential Revision: https://we.phorge.it/D25191

src/applications/transactions/storage/PhabricatorApplicationTransaction.php

@@ -1033,12 +1033,22 @@ abstract class PhabricatorApplicationTransaction
             count($rem),
             $this->renderSubscriberList($rem, 'rem'));
         } else if ($add) {
+          if ($this->isSelfSubscription()) {
+            return pht(
+              '%s subscribed.',
+              $this->renderHandleLink($author_phid));
+          }
           return pht(
             '%s added %d subscriber(s): %s.',
             $this->renderHandleLink($author_phid),
             count($add),
             $this->renderSubscriberList($add, 'add'));
         } else if ($rem) {
+          if ($this->isSelfSubscription()) {
+            return pht(
+              '%s unsubscribed.',
+              $this->renderHandleLink($author_phid));
+          }
           return pht(
             '%s removed %d subscriber(s): %s.',
             $this->renderHandleLink($author_phid),