So !258 and
!259 even would
have made things worse, thank you @eighthave for stopping me.

Before de73acf8 one feed was served at /feed.xml and the other one
at /feed (note it's a file, not a directory).
Instead of messing around with Apache to let it serve /feed/index.xml at
/feed/, I decided to just go with /feed.xml and sleep well.

So this change does the following:
* Remove any jekyll_redirects from feed.xml
* Serve Atom feed at /feed.xml
* Redirect from /feed to /feed.xml
* Set /feed.xml in all proper places