[floatplane] feat: share playlist post fields with playlist entries

[chtk]

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Description of your pull request and other information

Floatplane post can contain multiple items of video/audio, not just one. I'll call these playlist posts as opposed to regular posts

The extractor properly picks up all the entries in a playlist post. Downloading works fine. However some of the fields that would be available when downloading a regular post were not available for playlist entries.

I ran into this myself when I tried to use %(release_date)s in an output template.

This change:

• collects common fields for the post entry and playlist entries ...
• ... and spreads them in 1) the post info, and 2) every playlist entry
• adds a test to verify.

Fixes #

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