Press This: Add Preview functionality

[michael-arestad]

This was brought up earlier today and I think could make a good addition to the Action bar at the bottom.

Super quick mockup:

We need to discuss what happens when clicked. I think there are two options. One is more pleasant (and probably trickier) than the other.

1. The preview opens in a new window. This is pretty standard behavior, but is super rough when you have multiple tabs open or are on mobile.

2. The preview opens in a full screen iframe modal. The modal would have a "Close" or "Hide preview" button over it on the top right. This would enable quick edits and toggling of visibility without ever leaving the Press This window. Quick (ultra rough) mockup:

Thoughts?

[azaozz]

In 31654:

PressThis: add preview functionality. Opens the preview in a new window or a tab next to the source tab.
Fixes #31458.

[azaozz]

In 31655:

PressThis: make the Preview button "subtle".
See #31458.