Added configurable welcome content for Phabricator home page.

Summary:
The welcome.enabled feature is turned on by default; unsure of
whether this is desired by upstream.

Test Plan:
Apply the patch and see the welcome content. It's configurable
in the config.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5339