Add 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~
Added a deprecation warning against the plugin model render_template attribute, so we can remove it in future versions