Slack auth: Switched from community to laravel library

Slack auth: Switched from community to laravel library

Tested locally before & after change, and looked at code to compare.
Nothing seen or experienced that should affect things, from testing all
is working as expected with no difference from before.
- Update composer requirement of socialite to that which included slack.
- Updated PHP depds while there.
- Updated format of socialite events to align with current documentation
  and to use class references instead of strings.