Change layout and css

move #account_crud css to be above account info in app/views/accounts/show and
change 'margin-left: auto; margin-right: auto;' to 'margin: 0 auto;'
Update Gemfile.lock
Add display of average bill to accounts#show
Have average bill display 0 when no bills exist
Fix showing average bill if no bills exist. Also, show both invoice number
and current due for bills in account#show
fix missing link to invoice number for bills in account#show