Login/Signup buttons: update colors when appearance changes

[ScoutHarris]

Ref #12441
Auth PR: wordpress-mobile/WordPressAuthenticator-iOS#199

This uses the Auth changes that updates the Login & Signup NUXButtons when light/dark mode toggled.

NOTE: These buttons are not NUXButtons, and therefore are not fixed:

• Sign in with Apple. This will be updated on Update social login buttons appearance #13620.
• 'Connect another site' button on the login epilogue. This button will be removed with the new epilogue Unified Auth: new Login Epilogue #13413, and thus won't be an issue soon.

To test:

• Log out of the app.
• On the initial view, toggle light/dark mode.
• Verify the buttons update accordingly.

• Select 'Log In'.
• Toggle light/dark mode.
• Verify the buttons update accordingly (except for Apple).

• Select 'Sign up'.
• Toggle light/dark mode.
• Verify the buttons update accordingly (except for Apple).

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[Gio2018]

Looks good!! As usual please switch the pod reference to 1.11.0-beta.4 in the Podfile as soon as it's published.

[Gio2018]

👍