Allow border radius if only one control in field, fixes #1656

[jcs224]

This is an improvement.

If there is only one child inside a field with addons, the current first and last child selectors remove any radius. Since a single control would be both a first and last child at the same time, it gets canceled out. Fixes #1656

Proposed solution

This solution leverages the :only-child property to not apply this rule if there is only one control within a given field with addons.

Tradeoffs

None that I an think of, other than it may suddenly add radius to a .control only used once under a .field.has-addons.

Testing Done

I've tested this feature. Here is a field group:

And here is what this same field group with only one control looks like:

Notice the missing radius? This is what this fix does, before and after with one, two, three buttons:

With the addition of the :not(:only-child) selector, the :first-child and :last-child rules are ignored when there's only one child.

[My1]

awesome. thanks.