Refactor and include SessionUserConcern in app/controllers/application_controller.rb

We include SessionUserConcern in a handful of places

Should we just include it in app/controllers/application_controller.rb so it's available in all controllers?

I also think it would be nicer if session_user was a struct rather than a hash (so we do session_user.id instead of session_user[:id])

Implementation plan

1. Agree the above plan makes sense
2. Remove all separate includes of SessionUserConcern
3. Include SessionUserConcern in app/controllers/application_controller.rb

This could be as separate MR or part of the same:

1. Convert SessionUserConcern to a struct
2. Update all references to session_user[:x] to session_user.x

/cc @gitlab-org/developer-relations/contributor-success