Use Pouchdb for the cache

[claudioc]

The current cache for the navigation tree only works on a single node, since the cache is in memory. A better cache approach is to use Pouchdb directly to store it

The idea is valid only if the document tree is the same for all the users (which is the current assumption anyway). If that's not the case, then the cache should be per-person (and for the anonymous user)

(see /docs/CACHE.md)