Show event local time with local_time gem

Pivotal Tracker story *Show event page times with user's time zone and UTC* https://www.pivotaltracker.com/story/show/74592424
- Local time for events displayed in 'show' view using [local_time gem](https://github.com/basecamp/local_time)
- Local time determined by `current_occurrence_local_time` helper
- Test for local time added to spec/views/events/show.html.erb_spec.rb

Not directly related:
- Replaced hard-coded '(UTC)' with `%Z` in app/helpers/event_helper.rb (e.g. using 'UTC' was wrong for seeded events)
- Replaced `time_zone 'London'` with `time_zone 'UTC'` in db/seeds.rb & spec/factories/events.rb. The 'London' timezone goes to BST (UTC+1) in the summer, while 'UTC' does not change.