Add Basic Layouts for Devise's Auth Pages

[julien731]

We're using Devise for every project, and every time we're creating sign-in and sign-up pages that are quite similar in style.

In order to implement faster, we should have a basic layout defined in the template directly which could then be styled for each project.

[olivierobert]

Actually, the direction was to make more modular using mountable engines, hence https://github.com/burigems/buriauth.

So I am not in favor to add views templates as-is in the main Rails template.

An option is to make this optional using branches (as per the Android template):

So the flow, in this case, is to clone the main template and merge the extra UI branches on an as-needed basis.

[julien731]

Sounds good to me. We can then move this issue to https://github.com/burigems/buriauth