Ignore conflicts with events marked as "available"

[jplitza]

⚠️ This issue respects the following points: ⚠️

• I agree to follow Nextcloud's Code of Conduct.

Describe the goal you'd like to achieve

As a poll participant, I want to be able to only see events that actually block my time, because it helps to see which options I should check.

Describe possible solutions

Only show events as conflicts that are not set to show as "available" time.

Additional context

In the VEvent representation, this is distinguished by the property TRANSP:TRANSPARENT for events which are marked as "available", and TRANSP:OPAQUE for events which are "busy". It could be possible neither is set, which should IMO be treated as "busy".

Examples for events that are in a calendar but are shown as available could be holidays.