Add warning for when user tries to create possible duplicate

[aleksisalonen]

Users should be warned if they are about to create an event that already exists in the API.

This could be accomplished by doing a match fo the name field(s). Check for those name fields that the user has filled.

The check should only be activated in the background after the user has moved on to another field. Only future events should be considered.

In case of a positive match the blue bottom dialog should be shown, with the text:

Rajapinnassa on jo ainakin yksi tuleva samanniminen tapahtuma. Varmista ettet luo kaksoiskappaletta [link to event]tarkastamalla tapahtuman tiedot[link].

The link should take the user to a new new tab with search results for the event name.

Te dialog should be closable with an X button.