Implement user "bio" field

Perhaps this should be an extension, but it is pretty essential and I
can’t think of many instances where it wouldn’t be wanted. Would be
very easy to extract later on if need be.

src/Api/Actions/Users/UpdateAction.php

@@ -22,7 +22,7 @@ class UpdateAction extends BaseAction
         // second one failed, the first one would still have succeeded.)
         $command = new EditUserCommand($userId, $this->actor->getUser());
         $this->hydrate($command, $params->get('users'));
-        $this->dispatch($command);
+        $user = $this->dispatch($command, $params);
 
         // Presumably, the user was updated successfully. (The command handler
         // would have thrown an exception if not.) We set this user as our

src/Api/Serializers/UserSerializer.php

@@ -30,6 +30,7 @@ class UserSerializer extends UserBasicSerializer
         $canEdit = $user->can($actorUser, 'edit');
 
         $attributes += [
+            'bioHtml'          => $user->bioHtml,
             'joinTime'         => $user->join_time ? $user->join_time->toRFC3339String() : null,
             'lastSeenTime'     => $user->last_seen_time ? $user->last_seen_time->toRFC3339String() : null,
             'discussionsCount' => (int) $user->discussions_count,
@@ -40,6 +41,7 @@ class UserSerializer extends UserBasicSerializer
 
         if ($canEdit) {
             $attributes += [
+                'bio'         => $user->bio,
                 'isActivated' => $user->is_activated,
                 'email'       => $user->email,
                 'isConfirmed' => $user->is_confirmed