Use $global-radius as the default setting for side-radius()

Use $global-radius as the default setting for side-radius().
This way you can:
```scss
div {
  @include side-radius(bottom);
}
```
Instead of:
```scss
div {
  @include side-radius(bottom, $global-radius);
}
```
Merge pull request #2677 from MekZii/patch-1

Use $global-radius as the default setting for side-radius()