[UI] Add a setting for Sidebar position

[senwang86]

Summary

We used to talk about giving users an option to put the Sidebar on the left or right side, feel like it's ready to provide this option after PR #521

• Add a setting for the Sidebar position
• Correctly adjust the UI style properly based on the Sidebar position

Test

[lihebi]

This PR is cool. I was not able to make the left work in #521 due to flex displaying issues. Your approach looks nice.

[lihebi]

Resolved conflicts in #527 and merged. Closing. Thanks!