[UI] Add ability to download query plans in dot format

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I would like to be able to download query plans in dot format from the UI so I don't have to manually make API calls.

Note that eventually we will want to support rendering the query plans directly in the UI (#275) but this feature will be useful even when we have that.

Describe the solution you'd like
The scheduler UI shows a list of jobs and there is a "download" icon already. I would like to click on this icon to download the query plan in dot format, by making an API call to /api/job/{job_id}/dot.

Describe alternatives you've considered
None

Additional context
None