Validate that amount of credit to buy is in valid range

Fix sentry issue SGX-JMP-4

Typo not caught by tests because this method had none.  Add a test, fix the typo.

Refactor BuyAccountCreditForm

add_to_form no longer needs a promise or a network call, instead we use
a factory to get that data up front and inject the dependency.

New BuyAccountCreditForm#parse to get the relevant data back out of an
XMPP form.

Some changes to Transaction and sgx_jmp.rb to use the new method
semantics.

Split logic out into testable objects

Rubocop clean. Good test coverage of helper objects.