Show only enabled providers on the account link screen

Summary:
Ref T1536. Above, we load all providers, which is intentional (if a user has a link with a previously-enabled but now-disabled provider, we should enrich it with provider information).

However, before showing linking options we should drop disabled providers.

Test Plan: Disabled Disqus, reloaded, didn't see Disqus anymore.

Reviewers: mbishopim3

Reviewed By: mbishopim3

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6219

src/applications/settings/panel/PhabricatorSettingsPanelExternalAccounts.php

@@ -104,6 +104,8 @@ final class PhabricatorSettingsPanelExternalAccounts
         pht('Your account is linked with all available providers.'));
 
     $accounts = mpull($accounts, null, 'getProviderKey');
+
+    $providers = PhabricatorAuthProvider::getAllEnabledProviders();
     $providers = msort($providers, 'getProviderName');
     foreach ($providers as $key => $provider) {
       if (isset($accounts[$key])) {