Env `DEV_SERVER_LIVERELOAD` to disable webpack dev server livereload

What does this MR do?

Pickup DEV_SERVER_LIVERELOAD environment variable (set by the GDK) and enable/disable the webpack dev server inline livereload option appropriately, https://webpack.github.io/docs/webpack-dev-server.html#inline-mode

• Launch the GDK with DEV_SERVER_LIVERELOAD=false gdk run db

• Edit your GDK Procfile manually to add DEV_SERVER_LIVERELOAD=false before webpack-dev-server

• Add DEV_SERVER_LIVERELOAD=false to your .bash_profile

@mikegreiling https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9287#note_23738483

GDK MR: https://gitlab.com/gitlab-org/gitlab-development-kit/merge_requests/254

After the GDK MR is merged, in the GDK directory, echo false > webpack_livereload, gdk reconfigure, restart server gdk run

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

To disable livereload while developing

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #28204 (closed)