Allow assigning multiple groups to tasks

A single group can be assigned with group:

tasks:
  - name: Task
    duration:
      min: 20 minutes
      max: 33 minutes
    group: Backend

Multiple groups can be assigned with groups:

tasks:
  - name: Task
    duration:
      min: 20 minutes
      max: 33 minutes
    groups:
      - Team A
      - Backend

Tasks that have 2 and more groups require agents that belong to all listed groups, i.e. task's groups must be a subset agent groups.