This new site setting replaces the
`enable_experimental_sidebar_hamburger` and `enable_sidebar` site
settings as the sidebar feature exits the experimental phase.
Note that we're replacing this without depreciation since the previous
site setting was considered experimental.
Internal Ref: /t/86563
- better handling of drawer state using chat state manager
- removes various float and topic occurrences to use drawer
- ensures user can chat before doing a lot of chat setup
- fixes a bug which was creating presence errors in tests
- removes dead code
