Expose user tasks in GraphQL API

[saig0]

• import jobs with the user task job type as user tasks
• expose user task properties like assignee, candidate groups, and user forms
• expose user tasks:
  • via query
  • by key
  • of a process instance

⚠️ Jobs with the user task type are not exposed as regular jobs anymore. Instead, these jobs are only available as user tasks. Consuming applications may need to change their logic to see these jobs.

closes #284