Add size (or similar) prop to form input

Our design specs for inputs indicate that there are certain sizes (widths) that are allowed.

While the underlying BFormInput component already has a size prop, that changes the padding/margins around the input. In any case, our styles override that behaviour anyway. I suggest we repurpose the existing size prop to implement these widths.

The sizes are (based on the design spec):

Name	(Max) Width
xs	80px
sm	160px
md	240px
lg	320px
xl	560px

Questions

• What would be the default size? Currently, inputs take up all available width; i.e., they're unbounded. Should that still be possible?
• Is setting this via max-width sufficient?