Consider making "show template" option persistent

[annezazu]

Digging into the template preview feature during a recent hallway hangout, we discussed making this option persistent and potentially a user setting that could be set. In particular, for folks using screen readers, there’s no good way to explain what a template looks like when editing a smaller portion like a page. Being able to control whether this is on/off by default or make it persistent would help.

Here's the part of the call where we talk about this feature for context:

show.template.persistence.mp4

[spencerfinnell]

Worth keeping the post/page editors in mind and how they can utilize similar functionality: #27847