Add custom feed to show untagged posts

[RMarks]

On the Community team, we'd like to try and apply to each P2 post a Category and Tag. We already have setup a Slack feed to alert us about posts without a Category. This ticket is to add support for a URL such as ​https://make.wordpress.org/community/feed/untagged so we can get alerts about posts without a Tag as well.

I don't have the meta environment setup, so this is the best patch I can offer.

The code for the RSS template I found online. If there is a better template to use, there was nothing special about it after the <?xml> tag.

[RMarks]

A rough idea of a patch

[dd32]

In 11006:

Make: Add a plugin that generates a feed of all untagged posts.

Props RMarks for initial patch.
Fixes #5750.

[dd32]

In 11095:

Make: Prepend the custom feed to the title of the untagged feed.

Props RMarks.
See #5750.
Fixes #5801.
Merges ​https://github.com/WordPress/wordpress.org/pull/34.