Fix issue where drawer icon does not trigger the drawer to open on the feed page

[hjiangsu]

Pull Request Description

Review without whitespace

This PR fixes an issue where the drawer icon does not trigger the drawer to open. The solution here was to create a new scaffold key for the main scaffold containing the drawer (thunder_page.dart), and passing that down to the FeedPageAppBar to use.

This also consequently allows me to fix a visual issue where navigating to a community briefly shows the drawer icon rather than the back button. Navigating to communities should now show the proper icon on load!

This is related to #1014.

Issue Being Fixed

Issue Number: N/A

Screenshots / Recordings

Checklist

• Did you update CHANGELOG.md?
• Did you use localized strings where applicable?
• Did you add semanticLabels where applicable for accessibility?

[micahmo]

Thanks for finding and fixing this issue!

This also consequently allows me to fix a visual issue where navigating to a community briefly shows the drawer icon rather than the back button.

Sweet!