Can no longer do transfers with insufficient available funds.
Introduction of new DB Field in accounts: available_funds (type: double)
Field contains the funds that are currently available for transfer.

Necessary because of pending transaction approvals.
It is not possible to transfer an amount greater than the current value
of available_funds. Available funds is displayed below balance in user
UI.

Transactions that are approved (<= 10k): 
- available_funds and balance gets updated instantly.

Transactions that are not approved (> 10k):
- available_funds gets reduced by amount for source account
- no changes for destination account

Upon approval of transaction:
- balance gets increased by amount for destination account and reduced
by amount for source account
- available_funds gets increased by amount for destination account

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mybank-init.sql is a current dump of the DB
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2 test users exist (both PDF TAN users).
They have some funds and already some commitet transactions.
Email, PW
tattioff42+user1@gmail.com, Password1
tattioff42+user2@gmail.com, Password2
1 admin exists:
tattioff42+admin@gmail.com, Password0