When editing the Posts Page, the Template selector doesn't work

[jameskoster]

To reproduce:

1. Set a static homepage
2. Set a page to use as the "Posts page"
3. Edit the "Posts page"
4. Change the template in the "Template" panel of the document inspector to something custom (create a new template if necessary)
5. Save
6. Notice that the chosen template is not used

To be clear, I think it is appropriate that the template for this page cannot be changed, as that would allow folks to "break" the Posts page. However the appearance of the Template panel suggests that it can be changed, and the subsequent experience feels broken.

[jameskoster]

I don't know if this needs design. The simplest solution might be to replace the dropdown in the Template panel with some copy explaining why the template cannot be changed. The Edit link can remain. Rough mockup: