Web UI: add a profile page

We have the classics login/logout links at top-right in the Web UI, but we don't have a profile page (which is usually found in the same location after login). I think we could use one. For now we can have there the list of tokens and how to request them (possibly embedding or just link to /api/tokens). But I can easily imagine other stuff going there in the future, e.g., the contact email to receive notifications and a global and/or per-service notification toggle.

---

Migrated from T2718 (view on Phabricator)