Upgrade webpack to version v4.x (#6655)

Dockerfile

app/javascript/mastodon/common.js

app/javascript/packs/about.js

app/javascript/packs/admin.js

app/javascript/packs/application.js

app/javascript/packs/common.js

app/javascript/packs/public.js

app/javascript/packs/share.js

config/webpack/development.js

config/webpack/loaders/sass.js

config/webpack/production.js

config/webpack/shared.js

config/webpack/test.js

package.json

yarn.lock