Add PouchDB

[midsorbet]

This PR will only add the base service so that we can start to integrate PouchDB with Angular (related to #48)

• I did not move the couch service to the database folder because by doing that there will have to be a lot of one line changes to many files. I think it's better to do that when most of the services have been replaced by PouchDB.
• Angular 6 removed some polyfills because of which PouchDB was not working so I had to add some polyfills as suggested on PouchDB not working with Angular 6.0.0 pouchdb/pouchdb#7263

[paulbert]

I wonder if this approach of creating one database for PouchDB and storing all dbs from Couch here will cause similar problems that we saw with BeLL-apps of id conflicts. @dogi wondering what you think?

[midsorbet]

We could create one PouchDB instance per remote database if that's an issue.

[paulbert]

@lawrencek0 I was about to merge this, and then realized you were returning from Array.forEach a few times which is always undefined. I went ahead and removed the returns in 8254074, but if you intended to return a value from those functions you can change that in an upcoming PR.