Don't use magic numbers for LendingLimitTypes.
Use #sum instead of #inject.
Define a current scope on LendingLimit

Use the current scope to find the current lending
limits for a Lender.
Use SQL instead of filtering to return current lending limits.
Prevent offering a loan with an unavailable lending limit
Move logic to an UpdateLoanLendingLimit and corresponding controller
Update the loan's lending limit
Re-calculate the state aid figure when the lending limit changes
Lending limits have a 30 day grace period before they become unavailable
Change the lending limit for any completed loan

We should be able to change the lending limit, regardless
of whether the existing limit is unavailable.
Merge pull request #142 from alphagov/verify_lending_limit_at_offer

Verify Lending Limit at Offer