Customizer submit (activate/publish) flow

[jane]

When someone submits their changes through the customizer, they should be returned to the managed themes screen.

For modifications to the current theme, show yellow alert:
"Your theme customizations have been published."

For new theme activation, show the message we already show the next time the themes screen is loaded. "New theme activated. [widget prompt if applicable]"

[koopersmith]

In [20899]:

Theme Customizer: Improve activate and publish flow, make customizer states easier to track. fixes #20743, see #19910.