Switch out zeroclipbard for clipboard.js

Every browser except safari supports the api's necessary to support the "copy as curl" functionality without requiring flash or zeroclipboard.

[clipboard.js](https://zenorocha.github.io/clipboard.js/) wraps these apis and provides a fallback for safari which focusses an input box in the background and instructs the user to press command+c in order to complete the copy.

Start by locating the existing “copy as curl” entry point and its zeroclipboard integration; compare the browser API and clipboard.js behavior described in the issue. Done means the functionality no longer requires Flash or zeroclipboard, uses clipboard.js, and retains the Safari Command+C fallback.

Written by the indexing model from the issue text.