event details page responsive CSS work, added component and mixins

[dorono]

• refactored css for responsive behavior on event detail page
• created mixins for popover and breakpoints
• created component for social icons

NOTE: I put in a mixin to easily handle the breakpoints, like this:

  .right
    font-size: 0.8em
    display: flex
    flex-flow: row wrap

    @include breakpoint(md)
      flex: 1
      align-self: flex-start

After working on some projects with super-complex responsive behavior in the past, I've found it easier to keep the media-queries alongside the rest of the styles so you're not jumping all over the file to debug. Hopefully nobody minds this, and ideally, it will be useful.

I was able to get the arrow added to the popover, but it still needs the shadow added to the arrow part, but for now, I think it's worth merging in and coming back to if necessary.