Corpus header

Requires !630 (merged)

Closes #316 (closed)

This adds a header similar to the element details header to the corpora navigation. By providing an Actions menu, multiple buttons have been moved into a single menu in a component used both by corpora and elements, HeaderActions, and a significant amount of code was deduplicated.

This refactoring led to discovering #311 (closed), #316 (closed), #317 (closed), #318 (closed) and backend#347 (closed).