[WiP] POC implementation of get_render_template

A possible solution for #3225

`get_render_template` must not be defined in CMSPlugin so we can statically check if it's implemented or not, thus the use of the *private* `_get_render_template` as method used by the API
Naming is temporary.

Docs and tests needed
I don't like the idea of defining ```render_template``` in the plugin model as an option, there should be only one place to define this, otherwise it's going to get a bit confusing.