task-059: req-eng's tank balance UI

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+# req-092
+
+As a user, when I'm creating, updating and deleting my buckets and pipes, I want
+to see the result of the procedure,
+`calculate_tank_balance`, described below,
+so that I see a prominent number which I know summarises what the difference
+between my incomings and outgoings has actually been in the past, so that my
+incomings and outgoings are roughly equal in the long run.
+
+Source: req-091
+
+Satisfied by: dml-drip, dml-transaction, dml-bucket, ass-009, ass-021, req-093, req-094
+
+## calculate_tank_balance
+
+1. Let `drip_sum` be the sum of all `drip.amount` for all drips `drip`.
+2. Let `transaction_sum` be the sum of all `transaction.amount` for all
+   transactions `transaction`.
+3. Let `bucket_overdraw_sum` be the sum of all `bucket.balance` for all buckets
+   `bucket` such that `bucket.balance < 0`.
+4. Return `transaction_sum + bucket_overdraw_sum - drip_sum`.