Move templates into a _bergi subdirectory

[Duck Deux]

Our current directory tree is like this:

bergi/
|--- _bergi/
    |--- templates/
        |--- index.html
        |--- ...

The django docs suggests templates to be namespaced:

bergi/
|--- _bergi/
    |--- templates/
        |--- _bergi/
            |--- index.html
            |--- ...

One would also need to rename the references to partial templates, say, _nav.html to _bergi/_nav.html.