[Feature] Converting EuiPageTemplate to use ReactContext instead of cloneChildren

[cchaos]

Summary

Since most of Kibana's usages wildly separate the content from the template, the cloning method was not going to work as it required the children to be direct descendents without any file obfuscation.

So instead, I'm creating a template context in order to pass through the top-level props like padding and restricted width.

The exception of which is the sidebar. This needs to be a direct child because the presence of this exact element determines the configuration. I've noted this in the docs.

The bottom bar now uses createPortal to insert itself at the right place.

Checklist

• [ ] Checked in both light and dark modes
• [ ] Checked in mobile
• Checked in Chrome, Safari, Edge, and Firefox
• Props have proper autodocs and playground toggles
• Added documentation
• [ ] Checked Code Sandbox works for any docs examples
• Added or updated jest and cypress tests
• [ ] Checked for breaking changes and labeled appropriately
• [ ] Checked for accessibility including keyboard-only and screenreader modes
• [ ] Updated the Figma library counterpart
• [ ] A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_5987/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_5987/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_5987/

[cchaos]

@thompsongl This PR is good now, do you mind doing a quick once-over?

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_5987/

[cchaos]

Nevermind, sending this back to draft because the portal doesn't work when nested... 😖

[cchaos]

Ok, ready for review again 🤦‍♀️

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_5987/

[thompsongl]

Nice! This turned out well; LGTM

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_5987/

[cchaos]

Flakey?
Jenkins, test this

[cchaos]

CI's drunk, I need to move on, since this is merging into a feature branch anyway, I'm merging.