Add infinite earring

[StevenClontz]

Addresses #792.

[prabau]

Are you satisfied with the look on the web page (gigantic equation all by itself)? Also, we could be slightly more descriptive with something like:
"The subspace of ... defined by $X=...$, that is, the union of the circles centered at $(\frac 1n,0)$ of radius $\frac 1n$ for $n=1,2,\dots$."

Approving and leaving it up to you if you want to make some changes.

[StevenClontz]

we could be slightly more descriptive with something like:

I like this suggestion, thanks. And thanks for the proactive approval. Editing and merging.

[Jianing-Song]

@StevenClontz Add also that infinite earring is arc-connected and locally arc-connected, like S139?