Refactor ActiveRecord usage in dashboard controller

[chrisma]

app/controllers/dashboard_controller.rb

@notifications = Notification.where(user: current_user).slice(0, number_of_notifications)

loads all notifications and then discards. Instead: https://guides.rubyonrails.org/active_record_querying.html#limit-and-offset

[chrisma]

max_shown_vms and number_of_notifications might not be necessary.

Idea: Return all items and hide the majority in the UI...