Add placeholder classes to remaing utils

Status: **Opened for Review**
Reviewers: @kpeatt @ry5n @avelinet @cole-sanderson @mlworks @nastiatikk 
JIRA: **CSOPS-954**

## Changes
- Add placeholder classes (i.e. `$u-width-1of3`) to all utilities
Hmm, I think it ultimately came down me wanting to make the flex-initial fix be applied across all the dimension utilities while incapsulating the "fix" in a way that's easy to see in a single place.

Despite being a single property, at the time it felt cleaner to describe both the problem and solution in one place, and then extend it thus requiring no further comments or explanation.

The alternative would be to just add the property to each utility and use global line comments on all of them to describe the problem and solution. Ala...

```scss
// [a] When using flexbox blah blah... the fix is flex: initial, blah blah...

.u-width-x,
.u-width-y {
    flex: initial; // [a]
}

.u-width-a,
.u-width-b {
    flex: initial; // [a]
}

// etc.
```

The argument against the output CSS for both of these approaches is the same as the arguments for `@extend` vs. `@include`: more selectors vs. more properties.

I suppose you could add to the argument that by using a placeholder class, we're implying that it could be used elsewhere. Only we know that it is not intended to be reused.