Fix cascade issue with adding className to EuiPageContent

[Zacqary]

Summary

Fixes #2232

Reorders the insertion of className into <EuiPageContent> so that you can use class names to override properties otherwise specified by .euiPageContent--verticalCenter or .euiPageContent--horizontalCenter without using !important. The passed in className will now take precedence.

Checklist

• Checked in dark mode
• Checked in mobile
• Checked in IE11 and Firefox
• Props have proper autodocs
• Added documentation examples
• Added or updated jest tests
• Checked for breaking changes and labeled appropriately
• Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[snide]

This just needs a changelog entry. https://github.com/elastic/eui/blob/master/CHANGELOG.md

You can drop it under bugfix. Please used past tense.