Do periodic security update (yarn audit)

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Similar to issues in other repos:

• Do periodic security update (yarn audit) liferay-npm-tools#491
• Do periodic security update (yarn audit) liferay-js-themes-toolkit#504

This issue will substitute these currently open dependabot PRs:

• chore(deps): bump mixin-deep from 1.3.1 to 1.3.2 #1346 (mixin-deep)
• chore(deps): bump lodash.mergewith from 4.6.1 to 4.6.2 #1347 (lodash.mergewith)
• chore(deps): bump fstream from 1.0.11 to 1.0.12 #1348 (fstream)
• chore(deps): bump acorn from 6.1.1 to 6.4.1 #1389 (acorn)
• chore(deps): bump https-proxy-agent from 2.2.1 to 2.2.4 #1392 (https-proxy-agent)
• chore(deps): bump websocket-extensions from 0.1.3 to 0.1.4 #1399 (websocket-extensions)
• chore(deps): bump lodash from 4.17.11 to 4.17.19 #1408 (lodash)
• chore(deps): bump elliptic from 6.4.1 to 6.5.3 #1417 (elliptic)
• chore(deps): bump node-sass from 4.11.0 to 4.14.1 #1426 (node-sass)
• chore(deps): bump http-proxy from 1.17.0 to 1.18.1 #1430 (http-proxy)

As always, will want to be extra careful with the node-sass update as it is very sensitive to NodeJS version in the environment.

Note that in this repo, too, we applied new config to limit Dependabot to one open PR at a time — it will still "spam" us, in the sense that if we close that PR it can open another, but at least we won't have up to 10 open PRs in the list at any one time.

More context on our policy here: https://github.com/liferay/liferay-frontend-guidelines/blob/master/general/security.md

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Adding: #1433 (babel-eslint)

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Adding: #1434 (react-dom)

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Adding: #1435 (lodash, again)

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Adding: #1436 (parallel-webpack)

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Adding: #1438 (sinon)

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Adding: #1440 (mocha)

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Adding: #1441 (event-stream)

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Adding: #1443 (eslint)

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Adding: #1444 (webpack-dev-server)

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Adding: #1463 (sinon, again)

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Adding: #1464 (val-loader)

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Adding: #1465 (webpack-dev-server, again).

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Adding: #1468 (terser-webpack-plugin)

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Adding: #1470 (elliptic)

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Adding: #1471 (webpack)

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Adding: #1472 (gulp-sass)

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Adding: #1473 (prettier)

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Adding: #1474 (y18n)

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Adding: #1475 (webpack-merge)

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Adding: #1476 (karma-sauce-launcher)

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Adding: #1477 (del)

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Adding: #1478 (ssri)

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Adding: #1480 (bourbon)

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Adding: #1481 (karma-webpack)