Error template duplicates

Original discussion

!169039 (comment 2184684488)

Problem

GitLab uses templates to render error pages. Currently we have two places where they are stored:

1. public/* (for static templates)
2. app/views/errors/* (for dynamic templates)

That leads to a duplication of templates. We have to maintain two versions of them.

Proposal

1. Investigate why we do need public/* templates
2. Consider migrating to app/views/errors templates as a SSOT

Investigation

Looks like these static templates are used by NGINX. I found an old MR that introduced them: omnibus-gitlab!962 (diffs). When error code is 404, 500 or 502 the response from Rails app is replaced with a static page.