Reconfigure overwrites Mattermost settings made in System Console

• ZD: https://gitlab.zendesk.com/agent/tickets/47370
• Discussion: #1286 (comment 12349048)
• GitHub: https://github.com/mattermost/platform/issues/4178

The current file generated by omnibus is here: https://gitlab.com/gitlab-org/omnibus-gitlab/blob/4121e5853a00ed882a6eb97a40fc274f05d3b68c/files/gitlab-cookbooks/gitlab/templates/default/config.json.erb

There are a lot of settings that that can be specified in /etc/gitlab/gitlab.rb.

@it33's proposal:

We'd propose instead that omnibus backup and restore config.json and only make updates to the parameters set in gitlab.rb rather than overwrite the user's changes.

@marin's concerns:

This would actually mean us needing to parse the config.json file and then try to make a decision between the user values from the admin section, gitlab.rb file and the default. This gets complicated really fast. There needs to be a single source of truth.

We have more customers using Mattermost, so we should figure how best to resolve this issue.

I think we want Mattermost to manage most of the settings and GitLab provide only the essential settings, such as:

• GitLabSettings
• SqlSettings

Should we deprecate most of the settings in /etc/gitlab/gitlab.rb and find a way to get the settings into Mattermost? Thoughts?