WIP: Adding support for Nginx `proxy_set_header` for other nginx configs
What does this MR do?
This allows proxy headers to be configured on Gitlab Pages, CI, and Mattermost.
Are there points in the code the reviewer needs to double check?
The mattermost config is going to need some work/discussion. Potentially something like Gitlab#parse_proxy_headers
will need to be done.
I don't yet have a proper setup for mattermost, ci, and pages. I had to manually tweak the templates, to confirm the correct params were passed down to the templates. It should probably be tested on a VM with these settings enabled properly.
Why was this MR needed?
It was mentioned in #1304 (closed), that the tweaks should be added to all nginx configs.
What are the relevant issue numbers?
This continues the work on #1304 (closed).