fix: give the inline sign up form a max-width

[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 gives the inline sign-up form a max-width (along with https://github.com/Automattic/newspack-plugin/pull/3282 to add an extra CSS class).

I opted to add the styles in the theme because I'm not sure we're going to have the same setup/outcome with the block theme. Let me know if that doesn't make sense, though!

See 1205234045751551-as-1207890325042756

How to test the changes in this Pull Request:

1. Apply this PR and https://github.com/Automattic/newspack-plugin/pull/3282 and run npm run build
2. Go to /my-account and confirm that the sign-up form appears roughly the same size as it does in the modal.
3. Try signing in, and confirm that the different screens (OTP, password) look okay.

4. Log out and confirm the sign out message uses the max-width, too.

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?

[laurelfulford]

@kevinzweerink I could use your eyes on this just from a design perspective -- you can see what it looks like now by going to /my-account on one of the test sites.

[laurelfulford]

Thanks @chickenn00dle!

I stuck the styles here mostly in case we wanted to do something different with the block theme 🙂 But there's other ways to do that if it's better to keep them all together. I was a little on the fence about placement so am okay with moving them!

I'm going to spin up a fresh PR with both the plugin changes and CSS in one in the plugin and tag you for review just to confirm I haven't messed anything up! 😅