These changes should provide a more solid CSS approach to display the pay button in different scenarios. Also it fixes minor functionality issues like preselecting the merchant-chosen currency in the custom amount and slider dropdown.
* Start adding more options to Custom Amount Pay button
This allows you to simplify the custom amount pay button to remove the big + & - buttons along with set a min, max and step amounts. There's also an option to fit the button next to the input amount to have it more condensed(not finished)
* make fit button inline work nicely
* make currency dropdown more obvious
* fix space
