Fix and improve event filters / browse events search

[mariobehling]

The page https://eventyay.com/explore offers different ways to search for events. Please simplify the search options as follows:

• Add options show "All Categories" and "All Event Types"
• provide a tick box for filters
• move date filters to first position
• add filter option for Call for Speakers "Open", "Closed" and "Not Available"
• add filter option "Free" or "Paid"