fix #2707
Merge pull request #2709 from gratipay/fix-tip-button-2707

Fix position of tip amount suggestion
trailing_zeroes option added to format_currency, defaults to true
Removed trailing zeroes on support-gratipay banner
fix code style
simplify
go back to a simple `.replace()`
don't show trailing zeroes for user goals
don't show trailing zeroes for tip amount suggestions