[7.0] Add module 'customer_credit_limit'

I have reviewed modules 'customer_credit_limit' by Sistemas ADHOC and ECOSOFT and incorporated:
- A group that can approve a quotation, even when the credit limit is exceeded
- The possibility to manually run the credit limit check before the quote is attempted to be approved. This makes it possible for the sales user to run the credit limit check before printing and sending the quotation to the customer for approval. 
to be paid