This commit adds the redesigned button styles to the codebase along with
with a storybook-style page in `/devtools/buttons` to view and test the
redesigned button component.
The redesigned button component, uses the `action-button` class to
follow Zulip's no-abbreviation policy, and to avoid conflicts with the
pre-existing `button` and bootstrap `btn` classes.
A button using the new redesigned styles, required two classes,
- First, the base `action-button` class which defines the structure
and behavior of the button.
- Second, a modifier class like `action-button-primary-neutral` which
defines the styles for the particular action button type.
