feat(app) add navigation links from summary to manuscript and back

This is a version of !163 (closed) and pubsweet/pubsweet!195 (closed), based on the implementation of those MRs.
The key here is that I moved the App component into xpub-collabra itself.
The only difference in functionality is that both links are available on both pages, giving a bit of a tab-like feel when switching between the two pages.

If this goes through, we should close the other ones.
We should also delete xpub-app from pubsweet, as it was supposed to be there temporarily anyway.

Closes #114 (closed)

Note: there are some style issues, but let's solve those separately