show event menu bar.
add tabs_on_rails gem.
Optimization code #350
fixed rspec error.
Merge pull request #360 from saberma/add_event_menu_bar

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email participant fix #350
email participant job 2 fix #350
test code for email participant fix #350
merge with newest master code #350
Optimization code2 #350
Fix slim 2.0 compatible warnings
Merge branch 'fix-slim-2.0-warnings'
Extract Event#upcoming to ease test
Rename reminder_participants -> remind_participants
Merge branch 'feature/350-email-participant'