Adds persistent collapse button for left side bar
Created by: jasonblanchard
Fixes #8684 by adding a "collapse" and "expand" button on the left sidebar nav. State is stored in the session
hash as suggested by @randx.
The purpose of this change is to allow for more screen real estate for larger viewports.
Here's what it looks like expanded
Here is is collapsed (ahhh, so refreshing :)
The option is hidden on small viewports:
I'm not super in love with the button placement, so let me know if folks have other suggestions.
Note that I hit this bug https://github.com/activeadmin/activeadmin/issues/512 when testing the helper with the session
hash. I used the fix suggested in that issue which required that I changed a few other tests. Everything appears to be working locally, but let me know if it causes other problems.