Fix video count bug and re-enable default caching

(Note: already cross-merged to master)

Addresses a bug introduced in https://github.com/learningequality/ka-lite/commit/778a14c5074667ff534e8c2c67435612741414b9#diff-9efda8fb7be32a96fe044f464a17ba78L113 (deletion of the code to check and update video counts, in the topic context).

Fixes #871 and #873.