Switch to yarn to fix security vulnerabilities

- Replace all `npm` commands with `yarn`
The reason to do this switch is due to two reasons.
The first is that `npm install` completes in 1m30s, while
`npm install` completes in 50s, on average. The second is
explained in the point below. This runtime difference might appear
insignificant, but may improve development time over the long run.

- Remove `preinstall` script and change `resolutions` packages
The `preinstall` script is misleading and does not run before
dependencies are installed, and this is acknowledged in
npm/cli#2660 . With the switch to
`yarn`, the `preinstall` script becomes obsolete as `yarn` will
take care of the `resolutions` without needing a script. In
addition, the cause of the security issues was misattributed to
the wrong dependency in the previous commit. Remove this and add
the true culprits.

Dockerfile

@@ -2,13 +2,13 @@
 FROM node:alpine as client_builder
 WORKDIR /app/client
 COPY client .
-RUN npm install --production && npm run build
+RUN yarn install --production && yarn run build
 
 # => Build server
 FROM node:alpine as server_builder
 WORKDIR /app/server
 COPY server .
-RUN npm install && npm run build
+RUN yarn install && yarn run build
 
 # => Run container
 FROM nginx:alpine as base

client/package.json

@@ -19,11 +19,11 @@
   "scripts": {
     "start": "react-scripts start",
     "build": "react-scripts build",
-    "test": "react-scripts test",
-    "preinstall": "npx npm-force-resolutions"
+    "test": "react-scripts test"
   },
   "resolutions": {
-    "postcss": "^8.2.10"
+    "browserslist": "^4.16.5",
+    "glob-parent": "^5.1.2"
   },
   "eslintConfig": {
     "extends": [