5.8.0 release notes: clustering for early review

5.8.0 release notes: clustering for early review

- Update to group some code changes together

- Update 5.8.0.md

  Note - we put this out in 5.6.1 so I removed on purpose

  > - **Joomla admin theme menu, empty blocks & padding clean-up  ([12956](https://github.com/civicrm/civicrm-core/pull/12956))**

- Move items that are clearly features to features & elaborate

- sort report fixes

- move a line to wordpress

- group code cleanup fixes

- note  php 7.3 support as reason for relevant PR, merge post-fix into strtolower work