Add Slack as sign-in option to profile

Problem

In order to support the ability to unfurl GitLab links in Slack, from private projects, we need to be able to link the Slack workspace to the GitLab account.

Proposal

As a first iteration, we should start building out the OAuth flow on GitLab.com starting by adding a "Connect Slack" button to the user's profile/account page.

This OAuth flow will work similarly to the other social logins currently available today.