Add Slack authentication adapter

Summary: For phabricator. Adds a Slack auth adapater and icon.

Test Plan:
Create a new Slack Application for login, generate id and secret. Activate login and registration for Slack. Create a new account with Slack credentials. Log out. Log in with Slack credentials. Set my avatar with Slack. Slack. Slack.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D16496