Add sub items

[wookiehangover]

What does it do?

Adds a little form for creating sub items on stories.

Where should the reviewer start?

Yay for action-creators being reusable! This is really just a small addition to the SubItems component and some styling.

I also jimmied with some of the padding and layout in the item card detail to make things flow a bit better.

Background Context

Some flexbox magic in the form—that's becoming one of my favorite tricks.

Screenshots

[phoenixbox]

:+1: yay flexbox just working and feeling like :fireworks: