Consolidate S50 and S51 into S27 (rationals)

[prabau]

As it turns out, both S50 (Furstenberg topology) and S51 (p-adic topology) could be merged into S27 (rationals).

This is just the change to the S27 README file. Please let me know how it looks and I'll then commit the deletions of the other two spaces.

[prabau]

Closes #278

[marswill]

Looks good.

I think maybe for the second characterization, in terms of $T_3$, countably infinite, and first countable, it might help to refer specifically to section 5 in the arxiv reference (I didn't see it mentioned in the Dasgupta reference at all, maybe I missed it), and/or just give the one-line argument directly in the description. It is probably not so apparent to the casual reader why these characterizations are equivalent.

Also, while we're editing rationals, might make sense to delete not-pseudocompact and zero dimensional traits, as they are both redundant (you can get from metrizable and not weakly locally compact to not pseudocompact, and from metrizable and countable to zero dimensional, via various theorems on pi base).