Fix toggling DAG suspension for DAGs with custom names

[rocwang]

I find that toggling DAG suspension (i.e. "live") from frontend doesn't work when the DAG has a custom name defined:

Similar to #625, the frontend fix is extracting the file basename with out the .yaml bit as the DAG ID, and pass that to the backend for toggling.

However, upon fruther inspection, I also found the backend rest API doesn't return the right suspension status on a DAG with custom name. Furthermore, the scheduler also assumes a DAG's "name" is always the filename when calling client.IsSuspended(). So I fixed those as well.

It looks like DAG.Name is used quite a few times when actually a DAG.ID should be used instead. Maybe we should consider adding a new ID field to the DAG struct, which always stores the YAML filename, while DAG.Name can be overwritten with a custom name from the DAG spec.

[yohamta]

Thank you very much for fixing the problem! Yes, I totally agree that we should add the ID field or method to DAG and use it throughout the internal code.