build(deps): bump webpack-dev-server from 3 to 4

[halfwhole]

Problem

There's a high-severity Snyk security issue from using ansi-regex v2.1.1. This package is used by strip-ansi v3, which is in turn required by webpack-dev-server v3.

Solution

Upgrade webpack-dev-server from v3 to v4.

• This major version upgrade comes with breaking changes (see migration guide). So we have to change disableHostCheck: true to allowedHosts: 'all'
• Sadly, this upgrade doesn't seem to speed up the dev server :/ It's still slow

Tests

• Test that the dev server still works
• Test that staging still works

[gweiying]

lgtm! thanks for the upgrade