Allow use of `custom_gitlab_server_config` in Pages HTTP server
Allow use of custom_gitlab_server_config
in Pages HTTP server (e.g.
for specifying a custom location for LetsEncrypt's /.well-known/ folder)
The custom_gitlab_server_config
is required in the plain HTTP server
as the ACME's spec states that "the challenge must be completed over
HTTP, not HTTPS."
(https://ietf-wg-acme.github.io/acme/draft-ietf-acme-acme.html#rfc.section.8.3)