feat: add disabled button styles

[laurelfulford]

All Submissions:

• [x] Have you followed the Newspack Contributing guideline?
• [x] Does your code follow the WordPress' coding standards and VIP Go coding standards?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

This PR adds a disabled style for buttons.

The specific spot this is meant to address is the button on the password reset form. If your password is too short, the form displays a message and the button is disabled, but the latter looks the same as the regular button.

This PR fixes an issue reported in RAS-ACC, but it's also an issue on trunk, which is why I opted to fix it there.

See: 1207817176293825-as-1208436941710286

How to test the changes in this Pull Request:

1. Apply the PR and run npm run build.
2. As a reader, sign up for an account.
3. Go to My Account, and opt either to set or reset a password. You likely have to follow a link you receive in an email to get to the form:

4. On the password reset page, enter only one character in the first input. Confirm that you get an error for the password being too short, and that the button greys out:

Other information:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [ ] Have you written new tests for your changes, as applicable?
• [ ] Have you successfully ran tests with your changes locally?

[matticbot]

:tada: This PR is included in version 2.1.0-epic-ras-acc.4 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket:

[matticbot]

:tada: This PR is included in version 2.1.0-alpha.1 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket:

[matticbot]

:tada: This PR is included in version 2.1.0 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: