New dag run / task state: disposed

**Description** Introduce a new dag run / task state called "disposed". This new state would represent the acknowledgment of a failed run/task that should not be retried. Update the UI and CLI to provide a mechanism for the disposal action. **Use case / motivation** In a former gig we had a homegrown job management system. One nice feature for operations was the ability to "dispose" of a failed job. Disposal indicated that we recognized the failure, investigated it and decided the job should not be retried and the failure could be ignored going forward. This removed the failure from our daily operations report which we used to investigate failures. I find myself yearning for this feature lately. In airflow I have to either re-run the job or mark it as successful for it to leave my daily operations failure report. This could simply be implemented as a new dag/task state and action. We also had a "dispose reason" for tracking purposes - just a notes field for why the operator performed the action. Since we dealt with financial transaction feeds, we needed this. The auditability of a dispose state + notes field would be quite useful. **Related Issues** None that I see. Tagging @ryw