feat: make My Account button only show icon on mobile

[laurelfulford]

All Submissions:

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

Changes proposed in this Pull Request:

This either needs to be tested with Automattic/newspack-plugin#4443, or after it's merged.

See NPPD-1183.

How to test the changes in this Pull Request:

1. Apply this PR and feat[my-account-block]: add option to toggle off icon/text, and update tests newspack-plugin#4443 if needed.
2. Navigate to the Site Editor and edit the Header (Mobile) pattern; reset it first if it has any changes saved.
3. Confirm that the default pattern (pattern 1) uses an icon for the My Account Button block:

4. In the Template Part tab, switch the template part to use Header Mobile (Style 3). Confirm it also uses just the icon for the My Account Block button.

Other information:

• 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?

[thomasguillot]

@laurelfulford I updated the patterns based on the new classname approach.

I've also included a (future) fix (9322328). Related: #409