Don't show the "update" nag on initial installation of Largo

[rnagle]

It makes no sense that a user should have to update/upgrade after installing Largo for the first time.

We should see if we can:

• Use a theme activation hook to determine if this is the first time the theme has been activated (check for largo_version option or some other flag that is not present if the theme has never been installed)
• If this is the first time Largo has been installed and activated, set largo_version option to the current version

This should effectively prevent the update nag from displaying after installing Largo for the first time.

Related: #380.