Modify checkbox css to omit bullet/number and fix spacing

[vezwork]

Fixes #378

Checkbox items no longer have bullets or numbers (if in an unordered list or ordered list respectively).

Note: Checkbox items in bulleted and numbered lists look identical now. This could be misleading because if all the items are checkbox-ed, you won't know if its a bulleted or numbered list. I think this is okay because its consistent with how markdown is usually rendered.

[juliasilge]

This is looking so much better for un-nested lists!

However, nested lists still exhibit some problems, similar to what is pointed out in #378 (comment):

Here is what I was using:

-   [x] Ut enim ad minim veniam

-   [ ] Quis nostrud exercitation ullamco laboris

    -   [ ] Nisi ut aliquip ex ea commodo consequat

    -   [ ] Duis aute irure dolor

Are you going to address the nested lists with #378?

[vezwork]

I did remove the bullets/numbers for nested lists. I think what you might be getting at is that it feels like a checkbox that contains a nested list should not be vertically-centered beside that list, rather it should be vertically align to the top. I'll try to address that.

[vezwork]

@juliasilge I've changed the vertical alignment of the checkboxes. Does this address what you are talking about?

[juliasilge]

Yes, that looks great! 🙏