ikiwiki: FreedomBox URL is missing from RSS/Atom feeds

Problem:
The URLs in the RSS/Atom feeds generated by Ikiwiki only have relative paths to the blog/wiki.

Expectation:

• The feeds should have valid URLs.
• The domain name in the feed should always be current. Any changes to the domain name should be reflected in the feed URLs.

Detailed bug report on the forum:
https://discuss.freedombox.org/t/ikiwiki-rss-atom-feeds-not-valid/1298

Acceptance criteria:
After this bug is fixed, both the RSS and Atom feeds should be validated successfully by the W3C feed validation service.