Fix tx-push script for conference 2021 page
Fix tx-push script for conference 2021 page

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.

LLK/scratch-www,LLK/scratch-www