API design: Upcoming activities: Run scripts on online/offline hosts

[noahtalerman]

API changes for the "Upcoming activities: Run scripts on online/offline hosts" (#15529) story

Changes:

• Script endpoints are available in Fleet Free and Fleet Premium
• Update POST /scripts/run to add a script to the bottom of the upcoming activities (scripts are the only activity for now)
  • The new script will run after other upcoming activities
  • Failure of one activity won't cancel other activities
  • Remove validation for host is online
  • Remove validation for host is already running a script
  • Add validation for host already has the saved script as an upcoming activity. Can't add the same saved script to the queue
• Update POST /scripts/run/sync
  • Add validation and error if the target host has any upcoming (queued) activities
• Add GET /hosts/:id/activities to show past activity feed
• Add GET /hosts/:id/activities/upcoming to show upcoming activity feed
• Move docs for GET /hosts/:id/scripts to a new "Get host's scripts section" under "Hosts"

[noahtalerman]

Marking this PR as ready for review because we are about to ship the "Upcoming activities: Run scripts on online/offline hosts" (#15529) story