Katetov's non-normal subspace of $\beta\mathbb{N}$

[Moniker1998]

#360

[prabau]

github is complaining "The head ref may contain hidden characters ...", due to Katetov's name in there. But at least pi-base accepts the name for the preview mode.

FYI for future PRs: (although it may work in this case)
In the past we have had cases for example where someone had put a vertical bar in the branch name, and it was impossible to do anything with git, because the vertical bar was interpreted in a special way. It's better to limit the name to ascii characters and some of the non-alphanumeric characters should be avoided.

[Moniker1998]

Someone added S214 so sadly I need to resolve conflicts now...

[felixpernegger]

Remove the second sentence, as it is mentioned as a meta property of extremally disconnected already.

Also I think it's better to write "$X$ is a dense subset of ..." instead of "It's a dense subset of...". The first one sounds nicer in my opinion and is standard terminology on pibase (I believe).
This grammar thing appears in a lot of your PR's it would be nice if you could change them accordingly.

[felixpernegger]

P6 can be removed (is automatically deduced)