Fixes input-group sizes

### The Problem

This LESS below, has no SCSS translation.

```less
.input-size(@input-height) {
    select& {
        height: @input-height;
    }
    textarea& {
        height: auto;
    }
}
```

This is due to the placement of the ampersand (&). It's an intuitive way of using it but SCSS does not support it.

Which led to the .input-size mixin from the LESS version being incorrectly implemented in the SCSS version.

(Only showing the 1st of 6 rules to make the point concise) The outputted CSS was:

```css
.input-group-lg > .form-control select.input-lg,
.input-group-lg > .input-group-addon select.input-lg,
.input-group-lg > .input-group-btn > .btn select.input-lg {
  height: 45px;
  line-height: 45px;
}
```

when it should have been:

```css
select.input-group-lg > .form-control,
select.input-group-lg > .input-group-addon,
select.input-group-lg > .input-group-btn > .btn {
  height: 45px;
  line-height: 45px;
}
```

### The solution

I have remedied this by creating an extra mixin.

**Note:** The outputted CSS puts each rule on one line, rather than a new line at the comma separations. This was a consequence of refactoring the repeated selectors into the `$nested-selectors` variable (I thought it better the source be clean).

This is my first pull request so let me know if I've done else anything wrong.