Upgrade widget options only from admin

[ryan]

Currently, loading the front page of the blog will result in widget options being upgraded. On high-traffic sites and MU installs with lots of blogs, this results in a lot of writes to the options table[s]. Upgrades should be done only for admin page loads (perhaps even only for widgets admin page). For front page loads, the widgets options should be upgraded in memory but not saved to the options table.

[azaozz]

(In [11074]) Upgrade widget options only from admin, props ryan, fixes #9629