Change flex-basis for EuiPageBody

[ryankeairns]

Summary

@sebelga reported an IE11 layout issue within Management (screenshot below) where the layout would break for larger screen sizes. After digging in, the cause was related to the EuiPageBody having a flex-basis: 0% value. Changing this value to 100% makes the page body center itself properly in IE11 while continuing to work in other browsers as well.

This change was tested in Kibana, Cloud, and EUI docs on IE11, Edge, Firefox, Safari and Chrome.

Notes: This issue was only observed on the Management views where there is a second side navigation and where there are kui column layouts (div containers) being used for sections within EuiPage. In this situation, IE11 displays the content in the center of section, however it does so from the left edge of the content when the flex-basis is 0%.

Checklist

• This was checked in mobile
• This was checked in IE11
• This was checked in dark mode
• Any props added have proper autodocs
• Documentation examples were added
• A changelog entry exists and is marked appropriately
• This was checked for breaking changes and labeled appropriately
• Jest tests were updated or added to match the most common scenarios
• This was checked against keyboard-only and screenreader scenarios
• This required updates to Framer X components

[cchaos]

Thanks for taking the time to test it in all those environments!

[thompsongl]

LGTM

[sebelga]

Great, thanks for the fix @ryankeairns !