eslint: Activate prefer-const rule and apply fixes

assets/config/.eslintrc

...webpack-plugins/dump-highlightjs-languages-data-plugin.js

...fig/webpack-plugins/fix-swh-source-maps-webpack-plugin.js

...ebpack-plugins/generate-weblabels-webpack-plugin/index.js

assets/config/webpack.config.development.js

assets/src/bundles/admin/deposit.js

assets/src/bundles/admin/origin-save.js

assets/src/bundles/auth/index.js

assets/src/bundles/browse/origin-search.js

assets/src/bundles/browse/snapshot-navigation.js

assets/src/bundles/browse/swhid-utils.js

assets/src/bundles/origin/visits-calendar.js

assets/src/bundles/origin/visits-histogram.js

assets/src/bundles/origin/visits-reporting.js

assets/src/bundles/revision/diff-utils.js

assets/src/bundles/revision/log-utils.js

assets/src/bundles/save/index.js

assets/src/bundles/vault/vault-create-tasks.js

assets/src/bundles/vault/vault-ui.js

assets/src/bundles/webapp/code-highlighting.js