chore(deps): Get audit-filter working for all packages in monorepo

* Add a lint:deps job to the top-level package.json, so lerna can run
  lint:deps in all packages in parallel.

* Also fix today's handlebars vulnerability, so that builds don't fail.

Some of the vulnerabilities are in transitive dependencies, yet the
suggested `npm update foo --depth N` command sometimes seems to do
nothing. There was a related bug in npm 6.6.0 - 6.11.2, fixed by
npm/cli#239, but perhaps that didn't fix all the
cases? As a workaround, I've added exceptions where npm wasn't able to
fixup vulnerabilities.

Fixes #2229.

package.json

@@ -6,6 +6,7 @@
     "authors": "git shortlog -s | cut -c8- | sort -f > AUTHORS",
     "postinstall": "_scripts/install_all.sh",
     "npm-ci-all": "lerna exec --parallel -- npm ci",
+    "lint:deps": "lerna exec --parallel -- npm run lint:deps",
     "start": "pm2 start mysql_servers.json && echo \"Use 'npm stop' to stop all the servers\"",
     "stop": "pm2 kill",
     "start-firefox": "./packages/fxa-dev-launcher/bin/fxa-dev-launcher",

packages/fxa-amplitude-send/.nsprc

@@ -0,0 +1,8 @@
+{
+  "comment_535": "535 is a moderate severity RegExp denial of service vulnerability in mime, a dependency of s3.",
+  "comment_1118": "1118 is a critical severity arbitrary code execution in eslint-utils",
+  "exceptions": [
+    "https://npmjs.com/advisories/535",
+    "https://npmjs.com/advisories/1118"
+  ]
+}