[bp] Fetched correct PERP balance and added iter_history for PERP accounts

Customers complained the account.balance for PERP accounts was only the
"Rente annuelle estimée" instead of the total amount of money added to
the account. This patch implements the correct balance for PERP accounts
and repairs the bug in iter_accounts caused by the student_loans.
I also implemented iter_history for PERP accounts since they use a
specific page class.

Closes: 6974@zendesk