Like we were supposed to do three versions ago

* Replace mock-test with actual test

* Cleaned up .gitignore a little bit

* Added a .gitignore to modules, ignoring all non-tracked modules.

* Fix internal markdown links in the install docs

Ass^2

I'm such an ass 💩

Allow themes to inherit module templates

Migrate logout to twig

It doesn't make much sense anyway. Makes the original templates harder to format, and browser developer tools can provide hierarchical indentation anyway. This allows us to save quite a few bytes per page, write cleaner templates, and forget about how it would look like in the resulting HTML.

This is no longer required in HTML 5.

Non-JS version still needs work. We need to review also non-iframe logout.