Spec for `BigNumberWidget` Component

[gggritso]

Catch-all issues for work related to specifying and extracting the first Dashboard Widget Component: "Big Number". BigNumberWidget is the first and simplest one, so it only needs a specification for a handful of features for now.

Basic Features

• [x] Layout
• [x] Data
• [x] Visualization
• [x] State

Project Details Features

• [x] Previous Period Data

Dashboards Features

• [x] Thresholds
• [x] Full Screen View
• [ ] Banners