Add AccountService for adding/removing/activating accounts and store account data using JSON file

[gaviny82]

• Consolidate all logic for managing accounts into one class
• Fix #137
• Store account data in settings/accounts.json in local data storage
• Rename CurrentAccount to ActiveAccount
• In the settings service, only the UUID of the active account is stored. This can be used to find the account from accounts.json

Future work: build an AuthenticationService for adding accounts