Fix "New session" deck stats for installations with Postgres database

Depending on the used database, boolean attributes in the JSON
representation returned by the API can be of type integer (`0` or `1`)
or boolean (`false` or `true`). Code like
`answer_choice.is_correct === 1` does only work for the integer case
(Sqlite / MariaDB, where booleans are stored as integer / tinyinteger)
but not for `true`/`false` (Postgres, with a distinct boolean type). Fix
the definition of the reactive variables by checking for truthiness /
falseness instead of explicit integer values.

Resolves #819

Author: schu