Org-roam major redesign

Major changes:
org-roam-mode is no longer a global minor-mode: it’s the mode used for org-roam buffers. To set up org-roam, use org-roam-setup.

The test function currently used is org-roam-buffer. It should show you a buffer that looks like this, on a node:

Most parts of the buffer should be navigable now: You can jump to node, preview, olp, and unlinked reference. Preview UX is still WIP, but I don’t think it’s essential for v2.

Basically everything else will break: org-roam-bibtex, org-roam-server included.