* Add LIVEBOOK_SHUTDOWN_ENABLED
Previously we relied on code:get_mode(), but since
releases are starting in interactive mode to use
less memory, we need a clearer mechanism.
* Move shutdown feature check to Livebook.Config
Co-authored-by: Jonatan Kłosko <jonatanklosko@gmail.com>
* Run in interactive mode: it uses less memory and improves
boot time, which is the most desired properties for a dev tool
* Do not use releases/cookie, instead generate one on boot
