https://gitlab.com/gitlab-org/gitlab/-/merge_requests/80196 appears to
have broken development users who have configured their GDKs to use
HTTPS. Unless Webpack's `devServer.webSocketServer` is set to `false`,
a Websocket connection attempts to connect to an HTTP
endpoint. However, for security reasons browsers block mixed content
(https://support.mozilla.org/en-US/kb/mixed-content-blocking-firefox).

To fix this, we make it possible to enable and configure HTTPS via the
following environment variables:

* DEV_SERVER_TYPE
* DEV_SERVER_SSL_KEY
* DEV_SERVER_SSL_CERT

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/352648