Add impact severity to each violation

[MelSumner]

To help prioritize rules to be written or issues to be fixed, each existing detail needs a severity level added.

Severity should be based on impact to the user:

• blocker (no workaround, would cause loss of service or loss of critical functionality)
• critical (functionality is still available but is severely limited; no acceptable workaround is available)
• major (reasonable workaround available; no user-generated data would be lost)
• minor (degradation of non-critical functionality; workaround may be available)

To complete:

• [ ] add severity to the attributes list in lib/content-generator/index.js
• [ ] add severity to the front-matter list in blueprints/violations/files/details__name__.md
• [ ] add severity to each existing violations file with the correct severity indicated
• [ ] add a severity indicator in the templates/violations/violation file, and any supporting CSS necessary.

[MelSumner]

Another potential approach would be to take the same "weight" that Lighthouse gives each rule.