Modular template extension follows the master page extension

[jbanety]

This PR is related to the issue #1818.
This builds the modular template extension from the URI's extension (i.e. http://examp.le/my-page.amp or http://examp.le/other-page.docx).
It fallbacks to TEMPLATE_EXT const if URI extension is null or empty.

---

For my project :

I'm creating an AMP version of my website in the same theme.

my custom system.yaml

...
pages:
  ...
  types:
    - html
    - amp
  ...

So I have two template extensions : .html.twig for the regular pages and .amp.twig for the AMP pages.

I'm using the same content for the two versions.

If I create a modular page with the .amp.twig, Twig will load modular templates with the extension .html.twig (which will be wrong formated for AMP framework and not validated).

[rhukster]

Should this be limited only to modular pages? Does this make sense for modular and non-modular both??

[jbanety]

Modular template name is hard-coded with the constant.
So yes it's a quick fix for modular only.

[rhukster]

Ah yes, I see now.. Cheers.