Text buttons use opacity for hover

See merge request !5

This allows a text button to look appropriate in the App Bar when hovering over it, while still retaining desired hover behavior elsewhere by the UX design team.

I sourced the implementation from IconButton, since the hamburger menu icon in my app was behaving as desired.

Configure Rollup to ignore external dependencies

See merge request !4

Rollup is now configured to not bundle external dependencies. This
allows Material-UI to be used in the project while not inflating
bundle sizes or causing duplicated instances of `@material-ui/styles`
from being included in an app.

All of this was to support using the default theme's box shadow for
Contained-disabled buttons.

Continue adding IT Design System styles to buttons

See merge request !3

Use grays for outlined button border/hover, use Rollup to bundle

See merge request !2

Make form labels blue when focused

See merge request !1