Replace Bootstrap tooltips with CSS-only alternative

[thibaudcolas]

Our feedback form at the bottom of all pages uses Bootstrap tooltips to label the "smiley face" and "sad face" emojis.

We don’t use Bootstrap for anything else on the site. Let’s switch to a CSS-only tooltip technique, such as using the attr function on an after pseudo element’s content, so we can display good-looking tooltips without having to load 20kb of JavaScript