Knowledge Ranks at end of survey

Previously, when people reach the end of a survey we show them a "knowledge score" based on what percentage of features they know about or have used, along with a "knowledge ranking" showing the percentile in which they rank (e.g. "You knew about 90% of the features, which puts you in the top 13% of respondents").

The downsides are that:

- This knowledge ranking can actually be costly to calculate in the db
- It can also be unclear (not obvious that "top 90%" actually means "bottom 10%").

Instead, I was thinking we could just replace that with "knowledge ranks", for example:

- 0-10%: novice
- 10-30%: apprentice
- 30-50%: intermediate
- 50-70%: expert
- 70-90%: scholar
- 90-100%: elite

Maybe the rank could even be a combination of feature knowledge and quiz results (https://github.com/Devographics/surveys/issues/87)?

No files or tests are identified in the issue. Start by reviewing issue #87 and the current end-of-survey knowledge-score flow; clarify the rank bands and whether quiz results are included before defining completion criteria.

Written by the indexing model from the issue text.