Allow administrators to set a dashboard as a global default

Summary:
Ref T4883.

• When an administrator installs a dashbord, give them the option to install it as a global default.
• On the home page, if a user does not have a dashboard installed, check for a global default.
• On the Admin NUX/Welcome page, check for a global dashboard.

Test Plan:

• Installed a global dashboard, checked homepage, saw it.
• Installed a personal dashboard over it.
• Checked non-admin flow.
• Checked Admin NUX page for quest completion.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4883

Differential Revision: https://secure.phabricator.com/D9670