Theme Templates: Give users an option to "deactivate" a theme template.

[scruffian]

What problem does this address?

If themes ship with templates, it is impossible for users to opt out of them. For example if a block theme contains a front-page.html file then users have no way to stop this template from being used on the front of their site.

What is your proposed solution?

There should be a mechanism whereby users can opt out of a default theme template.

Here's just a rough design to get the ball rolling:

1. Adds a "deactivate" option to the template
2. Show inactive templates as grey.

[mtias]

This is an important gap to address. In the future, it'd be good to group disabled templates in a filter. The view when a template is disabled lacks information, though. It should probably read "disabled" where the ellipsis shows.