In CSV export, add `is-meta` column to `comments.csv`

[patcon]

Re-ticketed from #102 (comment)

This is currently not present in the CSV exports of upstream Polis, but is actually required in order to reproduce the calculations of Polis groups.

For a brief period, it was included in the exports, but was lost as a feature during one of the renewed product development pushes.

Example: https://github.com/compdemocracy/openData/blob/master/bg2050-volunteers/comments.csv

The above example also shows is-seed and group-aware-consensus as columns. These are not required to reproduce the calculations, but might be nice to add as well.