Add login/signup functionality to enable user management

## Expected Behavior

Having a login/signup functionality would enable data to be stored for different users.

## Current Behavior

Currently, the app for not up to the production level, a login/signup functionality would enable user management.

## Possible Solution

Try and use existing libraries for authentication and authorization or build/create of your own, whatever goes well with the app.

No files, tests, or entry points are named. Start by inspecting the Notes-app repository and its Apache Cordova structure, then determine how authentication and user-specific data should fit the existing app; done means users can sign up, log in, and have data stored separately.

Written by the indexing model from the issue text.