Redesign corpus header

Design nitpick:

I'm not a fan of the navigation between the search, model, and info page of a corpus. This is currently implemented as a toggle button to the right of the title:


This is mostly for historic reasons, as corpora initially did not have multiple pages, and so this addition preserved the overall layout of the page. However:

- It does not provide a clear visual hierarchy, or use conventional UI. Tabs would be clearer, for example.
- The icon for word embeddings is the best I could find at the time, but it's just not clear what it means. Text labels would be clearer (perhaps in combination with icons, *if* we can actually get a decent icon for the embeddings.)