Fixes#5912 : handling of the conference 2021 translation updates.
Testing:
* run `./bin/tx-push-www`:
* output shows what would run with the --execute flag.
* look for `tx-push-src` wtih resource name 'conference-index-2021-l10njson', for the conference 2021 view.
DO NOT RUN THE SCRIPT WITH `--execute`
Steps to land this PR:
1. rename the resource on transifex: `conference-2021-index-l10njson` to `conference-index-2021-l10njson` so that we don't lose any current translations.
2. merge the PR. Nightly update-translations job will execute the script.
3. rename or remove [conference-2021-index-l10njson](https://github.com/LLK/scratch-l10n/tree/master/www/scratch-website.conference-2021-index-l10njson) in scratch-l10n.
