This PR allows you to use the pay button generator to create buttons that target apps. This means that you can generate an invoice that is linked to an item on the POS/Crowdfund (targeting the item is optional). The POS/Crowdfund item amount -> invoice creation amount validation works too so that the user cannot modify the amount of a perk using just html ( fixes#1392 )
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
