Allow creating more specific templates from header dropdown

[mtias]

With #37407 partially accomplish, we should start looking at other relevant pathways to creating more specific templates. For example, from the "Archive" template you should be able to spin off a more specific date archive:

From the generic category template you should be able to create a more specific category-{slug} template, and so on.

Alternatively, or in addition to it, we could use the "template" sidebar panel for this purpose.

[jordesign]

Closing this issue as the top header dropdown is no longer used.