Event show page lists upcoming events both in UTC & local time

- events/show.html.erb: upcoming schedule changed to `<ul>`
  - each `<li>` element has class 'occurrence' for jQuery targeting and `data-event-time` in JS-parseable format
- event_helper.rb: `current_occurrence_time` changed to accept optional 'data' parameter to format output either for JS or for user-friendly display
- events.js: time and date parsed from `data-event-time`; converted to user's local time, with format determined by browser setting (`navigator.language` - not supported by IE :frowning:); appended to the `<li>` element as `<p class="eventLocalTime">`
- events.css.scss: eventLocalTime padded on the left to indent it relative to the original UTC time

I'll be happy to hear suggestions about both the implementation and presentation of local time on page.
Well, noted @yggie - I wanted to point this out, but completely forgot about it.
@NikitaAvvakumov we should pair on this, I will explain how to deal with Turbolinks.