Use standard selects instead of datetimepicker to select times

The component bootstrap3-datetimepicker-rails is now used only to select dates.
To select times in events/new and events/edit, we use standard select tags.

#refs 1226
Improve the summary of an event, specially the dates/time zone

Was showing the dates using the user's time zone but was also printing the
time zone used to create the event. Was confusing.
Not it still shows the dates in the user's time zone, but prints right below
which is this time zone in which the dates are being shown. We don't show
anymore the time zone in which the event was created.

Also made the entire summary and the social links a bit nicer.

refs #1226