Implement `queue chain` command

[sobolevn]

What needs to be done?

Blocked by #17

We need to implement this usecase: sometimes we need to queue multiple issues at once.
That's how it looks like:

@kira-bot queue chain
\```yaml
after: 10
chain: 
  - 14
  - task: 15
     chain:
       - 16
  - 17
  - task: 18 
     after: [14, 17]
\```

(note that \ is used for escaping purposes only)

That's what it should do:

1. You can work on 14, 15, and 17 in parallel
2. You can not work on 16 until 15 is resolved
3. You can work on 18 only when both 14 and 17 are resolved