refactor: add github-handle for Hyun Joo Sandy Oh (#8276)

We need to create a single variable github-handle to hold the github
handle for each member of the leadership team. Eventually github-handle
will replace the github and picture variables, reducing redundancy in
the project file.

In _projects/youthjusticenav.md, this is done by adding a key,
github-handle, to the leadership item, Hyun Joo Sandy Oh.

Author: myronchen-git

_projects/youthjusticenav.md

@@ -25,6 +25,7 @@ leadership:
       github: 'https://github.com/Rabia2219'
     picture: 'https://avatars.githubusercontent.com/Rabia2219'
   - name: Hyun Joo Sandy Oh
+    github-handle:
     role: UX Researcher
     links:
       slack: 'https://hackforla.slack.com/team/U03RZE7TTKP'