Improve date picking for event copy

[jeriox]

We are currently using django-recurrence for picking dates while copying an event. This has several issues:

• the JS code violates the CSP on every click because the library uses javascript:void(0)
• we cannot enforce an end date for recurrences
• the date picker is horrible
• there is no start date for a recurrence
• the picker for "every 14. of a month" is broken

We should either fix those issues in the library or write our own code. It would also be great to have the date preview updated after every change to the rules.