Implement right sidebar option as user option

Currently an admin can set the "sidebarRight" option, but the user is unable to change it for themselves.

This option felt like a natural fit, and I currently use it already on my instance.

CHANGELOG.md

@@ -10,6 +10,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Added quick filters for notifications
 - Implemented user option to change sidebar position to the right side
 
+
 ## [2.3.0] - 2021-03-01
 ### Fixed
 - Button to remove uploaded media in post status form is now properly placed and sized.