Create a side-effect free method to check db for username availability

[NiloCK]

User registration should provide feedback on the availability of usernames. Current implementation of availability attempts to load a db in a given name and read from it. This ends up CREATING this user-db - an unintended side effect.

Avenues:

• Require login for the creation of dbs (Not immediately obvious from Couch documentation)
• the skip_setup option on db creation may be what I'm looking for, but the workaround for PouchDb 7 : authentication doesn't work anymore pouchdb-community/pouchdb-authentication#239 is impeding the normal passing of settings to the pouch ctor.

[NiloCK]

Better approach:

From https://stackoverflow.com/questions/52269374/how-to-check-if-the-db-exists-in-couchdb-or-not:

You could evaluate the db name from de user name and then check the existence with this API operation:

HEAD /userdb-{hex encoded username}

If the users are managed by your application and are not defined in couch, you can just check the database existence using your db naming rule

In practice: Head requests fail w/ 401 - unauthorized if the name is taken, and 404 - not found if the name is available.