Draft: Guide user toward choosing a root disk size large enough to accommodate selected image

Fixes #645 (closed).

Plan

• Use larger of image size and min_disk property to determine minimum required root disk size
• When selecting a flavor with a root disk smaller than minimum, require volume-backed root disk
• If a volume-backed root disk is selected, set minimum backing volume size to whatever is required by the image

How to Test

(Please describe how to exercise these changes in the application)

Screenshots

(If visual changes are introduced, show what they look like)

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