Base for generic headers

!453 (closed) was a large and old merge request that could have been split in at least three merge requests. This is the first of those smaller MRs.

This does three things:

• Remove the named nav router view that was used for the ElementHeader (see js/router.js and vue/App.vue)
• Add SCSS code to have a <header class="header"> providing all the header styles (scss/main.scss)
• Have every single page use a main class="container is-fluid" to allow each page to have its header as needed (every other file)

This merge request will then make it easier to add a header to any page. However, the ElementHeader does not use this system yet as it should be fully refactored to split it into multiple components and move some of its data or props to the store.