Fix: Provide absolute URL for posts in RSS feed

As called out on Twitter[0], the links aren't valid as the are relative,
not absolute.

Closes #376.

[0]: https://twitter.com/MaximRouiller/status/1100417503027777536
5 jobs for defect/rss-feed in 5 minutes and 59 seconds (queued for 1 minute and 10 seconds)
Status Job ID Name Coverage
  Build
passed #167926031
build_deploy_image

00:00:43

passed #167926029
build_image

00:02:52

 
  Review
passed #167926032
review_deploy

00:01:05

passed #167926034
manual
review_stop

00:00:43

 
  Test
passed #167926035
test_image

00:01:18