R1:abcd3efa5a2f

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…