closes #158

• Adds an account table and links users and generic assets to it.
• adds CLI tasks to add/delete accounts
• many small items coming from this change, e.g. in the UI

Upgrading the database

Verify that the current revision is b6d49ed7cceb or higher before checking out this branch.

The upgrade should be straightforward:

flexmeasures db upgrade

This will:

• Make sure each user has an account, creating new accounts based on their email address domain.
• drop generic_asset.owner_id associations (and move them to generic_asset.account_id, using the accounts from step 1). This will be backed up locally, should you want to db downgrade later.

Note:

You can add custom user-account relationships, e.g.:

flexmeasures db upgrade +1 -x '{"1": "Seita", "3": "CustomerA", "30": "CustomerB"}'  

This would associate users 1, 3 and 30 to accounts with the names "Seita", "CustomerA" and "CustomerB", respectively, creating these accounts on the go.