[Cases] Add spacing between all the sections in the case view sidebar

There is inconsistent spacing between the various sections in the right hand side bar of the case view page. In particular the 'Reporter', 'Participants' and 'Tags' sections:

<img width="361" alt="image" src="https://user-images.githubusercontent.com/7405507/203348276-a1ad2d1c-5829-4564-9469-225353508c83.png">

This may have been introduced by changes to the `EuiFlex` layout in #144845.
