Remove active duplicates from Events API

[iandunn]

This is related to #2720 and #3027, but distinct.

There are cases where a meetup organizer will accidentally create duplicate events, but not realize it, and leave all of them active (instead of deleting the duplicates so that there's only 1 total).

For example:

• ​https://www.meetup.com/Puyallup-WordPress-Meetup/events/245787617/
• ​https://www.meetup.com/Puyallup-WordPress-Meetup/events/245788104/
• ​https://www.meetup.com/Puyallup-WordPress-Meetup/events/245787574/

That has the effect of pushing all other upcoming events out of the Events Widget.

When the official-wordpress-events plugin pulls new events from the meetup.com API, though, we can detect if they have the same title and date as existing events, and just ignore them, instead of saving them.

The most efficient way to do this may be to add a unique key constraint over multiple columns, rather than trying to do something in code.

[iandunn]

This is closely related to #3027 and #2720, so I think we should take those into account when considering solutions. There may be an elegant solution which solves all of those cases; whereas coming up with 3 independent solutions could get messy.

[iandunn]

This is mitigated by r6814, since cancelled events will not be shown any longer. Active duplicates are still shown, though.

Additionally, deleted events are still shown, see #3491.