Fix remaining feed validation errors

[simonw]

https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Fwww.niche-museums.com%2Fbrowse%2Ffeed.atom%3F_feed_title%3DNiche%2BMuseums

Sorry

This feed does not validate.

• line 3, column 77: id must be a full and valid URL: browse/cdd87adc6bf7c42d03283a5076f268161bd4d3a702e6643a1be32af787ca0d10 [help]

  browse/cdd87adc6bf7c42d03283a5076f268161bd4d3a702e6643a1be32af787ca0d1 ...
  ^

In addition, interoperability with the widest range of feed readers could be improved by implementing the following recommendation.

line 2, column 0: Missing atom:link with rel="self" [help]

<feed xmlns="http://www.w3.org/2005/Atom">

These should now be possible to fix since we can get the full request object after simonw/datasette#581

[simonw]

https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Fwww.niche-museums.com%2Fbrowse%2Ffeed.atom validates now!