Add date time picker for events

[Adda0]

This PR contains:

• date time picker for planning or modifying events,
• user-friendly messages instead of empty tables for events and linked states.

[ondryaso]

While you're at it:

• The Smazat button in the /events/plan form is kinda pointless
• Change Potvrdit to Přidat akci (or Uložit změny when editing)

[Adda0]

Will do both. Give me a moment.

[Adda0]

Done, plus some additional user-experience changes.