Update license user association from has one to has many

[ezekg]

Closes #534.

Prerequisites

• Add API endpoints for assigning/unassigning users to licenses. (Should we use attach/detach?)
• Update user validation scope to look at License#users.
• Add owner validation scope.
• Rename License#user to License#owner.
• Rename singular user relationship to owner in user serializer.
• Add plural users relationship to license serializer.
• Add migration to rename data.relationships.owner to data.relationship.user for licenses (with updated links).
• Bump API version to v1.6.
• Rename license.user.updated webhook event to license.owner.updated? (Or send both?)
• Write integration tests for license owner vs license user authz.

Pre-deploy

• Create license.owner.updated, license.users.attached, license.users.detached, machine.owner.updated event types.
• Create license.owner.update, license.users.attach, license.users.detach, machine.owner.update permissions.

Post-deploy

• Rename licenses.user_id to licenses.owner_id? (May want to do this during our next maintenance window.)
• Rename license.user.update permission to license.owner.update?
• Reseed database with new permissions/events.
• Add license.owner.update, license.users.attach, license.users.detach, machine.owner.update permissions to relevant roles.

[ezekg]

New plan: with the proof of concept union_of gem, we're able to create associations that union across multiple associations. This allows us to really clean up our implementation here, and gets rid of the need to do a multi-part PR.

Let's clean up union_of and add more exhaustive tests, and go in the direction mentioned in #534 (comment).

[ezekg]

Should a machine belong to an owner? Right now, it doesn't make sense that a machine always belongs to the owner of the license, since in reality, it could belong to any of the license's licensees, not necessarily the license's owner.