fix(webpack): set publicPath to / in dev mode

[tthvo]

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Related to #1175
Related to #1284

Description of the change:

Set the publicPath in webpack.dev to /, which configures the web assets path to /<asset-path> when using with hot-reload server.

This way, directly accessing sub-path such as /rules/create should work. It makes workflow easier when we are working on sub views (i.e. create rules, recordings, etc).

How to test

1. Start dev-server with yarn start:dev or yarn start:dev:preview.
2. Access the sub view, for example, /rules/create directly by entering the URL.
3. The view should load correctly.
4. Make some changes to those sub-view and verify that the sub view is reloaded correctly.