Add basic "Subscriptions" application

Summary:
Basic infrastructure for generalizing subscriptions/CCs for T1808, T1514 and T1663.

  - Implement `PhabricatorSubscribableInterface` and you'll get a subscribe/unsubscribe button for free.
  - If there are any auto-subscribed users (like the question author) you can specify them; this makes more sense for Tasks and Revisions than Ponder probably, but maybe the author should be auto-subscribed.
  - Subscriptions are either "explicit" (the user clicked 'subscribe') or "implicit" (the user did something which causes them to become subscribed naturally). If a user unsubscribes, they'll no longer be added by implicit subscriptions. This may or may not be relevant to Ponder but is an existing Herald feature in Differential.
  - Helper method on PhabricatorSubscribersQuery to load subscribers.
  - This doesn't handle actually sending email, etc. I think that's all so application-specific that it doesn't belong here.
  - Now seems to work.

Test Plan:
{F20552}
{F20553}

Reviewers: pieter, btrahan

Reviewed By: pieter

CC: aran

Maniphest Tasks: T1663, T1514, T1808

Differential Revision: https://secure.phabricator.com/D3637

scripts/celerity/generate_sprites.php

@@ -169,18 +169,21 @@ $action_template = id(new PhutilSprite())
   ->setSourceSize(16, 16);
 
 $action_map = array(
-  'file'        => 'icon/page_white_text.png',
-  'fork'        => 'icon/arrow_branch.png',
-  'edit'        => 'icon/page_white_edit.png',
-  'flag-0'      => 'icon/flag-0.png',
-  'flag-1'      => 'icon/flag-1.png',
-  'flag-2'      => 'icon/flag-2.png',
-  'flag-3'      => 'icon/flag-3.png',
-  'flag-4'      => 'icon/flag-4.png',
-  'flag-5'      => 'icon/flag-5.png',
-  'flag-6'      => 'icon/flag-6.png',
-  'flag-7'      => 'icon/flag-7.png',
-  'flag-ghost'  => 'icon/flag-ghost.png',
+  'file'              => 'icon/page_white_text.png',
+  'fork'              => 'icon/arrow_branch.png',
+  'edit'              => 'icon/page_white_edit.png',
+  'flag-0'            => 'icon/flag-0.png',
+  'flag-1'            => 'icon/flag-1.png',
+  'flag-2'            => 'icon/flag-2.png',
+  'flag-3'            => 'icon/flag-3.png',
+  'flag-4'            => 'icon/flag-4.png',
+  'flag-5'            => 'icon/flag-5.png',
+  'flag-6'            => 'icon/flag-6.png',
+  'flag-7'            => 'icon/flag-7.png',
+  'flag-ghost'        => 'icon/flag-ghost.png',
+  'subscribe-auto'    => 'icon/unsubscribe.png',
+  'subscribe-add'     => 'icon/subscribe.png',
+  'subscribe-delete'  => 'icon/unsubscribe.png',
 );
 
 foreach ($action_map as $icon => $source) {