i15-1: Allow submitting plans to the queue

Plans page should have some improvements:

- [ ] RunPlanButton should be disabled if the queue is busy
- [ ] Each plan should have an "Add to queue" button which can add the plan to the start or the end of the queue

Start at the Plans page and locate RunPlanButton and the existing plan actions. Check how queue busy state and queue ordering are represented; done means RunPlanButton is disabled while the queue is busy and each plan can be added explicitly to either the start or end of the queue.

Written by the indexing model from the issue text.