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