Add copy to clipboard buttons

[mad-gooze]

This pull adds copy to clipbord buttons for dev and production urls.

[rgrove]

Thanks for the contribution!

Normally I don't recommend implementing new functionality without first asking whether I'm interested, but in this case I'm open to adding a copy button. However, there are a few issues (mostly design-related) that will need to be fixed before I'd be willing to accept this PR:

• [ ] The button doesn't work in Safari, but gives no indication of this. Ideally the button should not be displayed at all in browsers that aren't supported (this should be done via a feature test if possible).
• [ ] The icon is blurry and fairly unattractive, especially on high-DPI screens.
• [ ] The icon isn't vertically centered within the button.
• [ ] The button's left border clashes with the right border of the input field, and the spacing between the two is inconsistent between browsers (Chrome vs. Safari, for example). It would be nice if the two elements fit together better.

Here's a screenshot of what the button currently looks like in Chrome, for reference:

[mad-gooze]

• [x] The button doesn't work in Safari, but gives no indication of this. Ideally the button should not be displayed at all in browsers that aren't supported (this should be done via a feature test if possible). Added checking using polyfill.
• [x] The icon is blurry and fairly unattractive, especially on high-DPI screens. Image is changed to Font Awesome icon generated via IcoMoon
• [x] The icon isn't vertically centered within the button.
• [x] The button's left border clashes with the right border of the input field, and the spacing between the two is inconsistent between browsers (Chrome vs. Safari, for example). It would be nice if the two elements fit together better.

[rgrove]

Thanks! I've deployed this with a few small tweaks (I used an SVG of GitHub's clipboard Octicon and changed the styling a bit).