base: separation of bootstrap bundle

Proposed solution for https://github.com/inveniosoftware/invenio/issues/1996
Sidenote, invenio.base.bundles is not part of `docs/base.rst` yet (why isn't it into `api`? anway). And you can use `.. py:data:: <variable>` to document a specific variable. It's usually nicer than the docstring following the variable declaration. E.g., [invenio.ext.assets](https://github.com/inveniosoftware/invenio/blob/4e14f8ec92062e7fda572ba329505eb8bbb53a3c/invenio/ext/assets/__init__.py)