Show resource details on instance details page

1. There is now a toggle tip next to instance size/flavor which shows detailed resource quantities belonging to the instance (CPU cores, RAM, storage). If there is at least one GPU (as specified in the flavor extra_specs), the toggle tip shows that as well.
2. Each of the resource usage graphs now show a sub-heading of the total quantity of that resource belonging to the instance.

In the quest to show an instance's root disk size when it is volume-backed, I factored out and centralized some volume-related helper code.

Fixes #637 (closed). I was going to attempt #646 (closed) here but decided to do that in !584 (merged) instead, so apologies for the slightly misleading branch name on my fork.

How to Test

Click the toggle tip, look at the graphs

Screenshots

QA Checklist

• Any changes to 'widgets' in src/Style/Widgets/? If so:
  • Update src/Style/StyleGuide.elm showing example usage of that widget
• Were any app flags created, modified, or removed? If so:
  • Update config.js and all files in environment-configs/, including docker-config.js
  • Update "Runtime configuration options" documentation in README.md

Post-Merge Checklist

• Create follow-up issues for anything left over for another merge request
  • If any technical debt is introduced, assign these follow-up issues to the MR submitter