Track how many times someone has taken previous editions of a survey

It would be nice to know which proportion of respondents have taken the survey a single time, twice, three times, etc. over previous years.

I could see different approaches:

### 1. track this in `surveyform` and submit it as part of every save

Pros: distributes the workload over every save
Cons: only works for future respondents

### 2. do it as part of the normalization process

Pros: will work for every response, even for past surveys if we renormalize them
Cons: will slow down normalization drastically (+1 db request per response processed)

### 3. do it as part of a `surveyadmin` script

Pros: since this is something that doesn't change, we can calculate it once per survey even if it takes a long time
Cons: easy to forget to run it