Add search feature to WordCamp currency dropdowns

[iandunn]

The wordcamp-payments plugin has several modules that show a dropdown list of currencies. There's about 165 in the list, so it's very difficult to navigate.

The two most commonly used currencies (EUR and USD) are placed at the top as a quick UX hack, but that's not a good solution because it doesn't help finding the other currencies.

We should implement a JavaScript <select> replacement, so that the list can be easily searched. It looks like Core is moving in the direction of ​select2 (per ​#wp31696), so let's use that.

[iandunn]

As part of this, it'd probably be good to create a render_currency_field() wrapper, instead of duplicating the Select a currency option, etc, in every instance.

[iandunn]

Select2 was added to the codebase in r6609. See get_wordcamp_dropdown() for an example of how to use it.

[vedjain]

In 7267:

WordCamp-Payments: Add select2 to most select fields for better UX

In budget-tools.js, there was also an undetachted even handler, which was causing rendering of select box multiple times. That is also fixed in this commit. This is mostly a UX change.

Fixes #3241
Props vedanshu