mattermost channel should show helpful text when linking to private repos

[ratancs]

A a receiver of a link, I see a sign in text when my colleague shares link of a private repo.

[ftab]

Also want this for our self hosted GitLab.

related to #212 ?

[spirrello]

Would be awesome if this was possible.

[mickmister]

This is currently unrelated to the gitlab plugin, as this is rendered by the webapp itself based on the open graph data we get from the unauthenticated request to GitLab. However, we could implement this in the plugin with https://developers.mattermost.com/integrate/reference/webapp/webapp-reference/#registerPostWillRenderEmbedComponent, by fetching this per-user at runtime. @hanzei What do you think?

[hanzei]

Implementing a custom render is a fine approach. I wonder if (additionally?) the server side API should expose a hook that allows plugins to provide an API token when fetching the post metadata. That fix would also work for mobile.