Clean up hardcoded URL on RSS feed

[jhonatan-lopes]

The RSS feed is defining the link and feed_url URLs hardcoded to the https://foundation.mozilla.rog/ site. This is a bad practice. We should aim to provide relative urls within the codebase:

class RSSFeed(Feed):
    """
    Blog page RSS feed, using the content:encoded serializer above
    """

    title = "Mozilla Foundation Blog"
    link = "https://foundation.mozilla.org/blog/"
    feed_url = "https://foundation.mozilla.org/blog/rss/"
    description = "The Mozilla Foundation Blog"

Using relative URLs, the syndication framework will construct an absolute URL for the feed using the current site.

┆Issue is synchronized with this Jira Task

[danielfmiranda]

Closing this issue as the fix has made it to prod vis #12228