Merge S50 into S27

[StevenClontz]

See https://math.stackexchange.com/questions/1849271/homeomorphism-between-evenly-spaced-integer-topology-and-the-rationals which I'll attempt an answer to (though it follows as a corollary to the usual characterizations of the rationals). (EDIT: never mind, got to be too tedious to be worth the time, but would be an interesting student question someday...)

[prabau]

Yeah, I knew about it, but tedious indeed. Sometimes it's useful to have two perspectives on a space, especially as the homeomorphism is not explicit.

[StevenClontz]

Agreed on usefulness of perspectives, unsure if the best way to handle it is having two IDs for the same space.

[prabau]

For reference, some precedents of combining two homeomorphic spaces from Steen & Seebach:

• https://topology.pi-base.org/spaces/S000028 combines counterexamples 31 (irrational numbers) and 102 (Baire space).
• https://topology.pi-base.org/spaces/S000002 combines counterexamples 2 (countable discrete topology) and 135 (Sierpinski's metric space).

[prabau]

https://math.stackexchange.com/questions/2328731 has some useful links for the topological characterization of $\mathbb Q$.

Also https://teratologicmuseum.wordpress.com/2009/05/05/a-metric-for-the-evenly-spaced-integer-topology/

[StevenClontz]

pi-base/web#134