Move content article element and header into `base.html`

We have to create an article element on every single page, and we have to set up a header with a title and (probably) breadcrumbs on every single page. This PR moves all of that into a new header block in base.html. The block can easily be extended with {{ block.super }} when you need more content in the header.

This PR takes the opportunity to rename some blocks.

The score management buttons are now displayed under the title, instead of on the same line. These buttons are an edge-case that I didn't want to set up a special solution for, since all management buttons are up for an overhaul in #120 (closed). They actually look really good in their new position...