Fix the decimal symbol in various places

This fixes how the numeric value in the tip input box is displayed, as well as a few places where the numbers are updated via JavaScript.
This isn't the right approach, the text still isn't properly localized: there is still "/wk", and the dollar symbol is still before the number (for languages like French it should be after). I see two possible solutions: have the server send back the fully localized text, or port Babel's functions to JavaScript so we can use them client-side.