Configurable (non hardcoded) oembed server

I would suggest the ability to add custom OEmbed proxy server. Actually latest mattermost code is using hardcoded noembed (see https://github.com/mattermost/platform/blob/bb69e98631b2541954b6ae465b8ba5f788b9dc49/webapp/components/post_view/components/post_attachment_oembed.jsx#L33).

Why do I want to customize url?

1. When company proxy or firewall block noembed and use your own hosted noembed
2. When noembed is KO
3. When noembed not support some providers (like spotify https://github.com/leedo/noembed/issues/79) and use your own fork or alternative solutions like iframely (opensource), embedly (maybe not support it in first place because depends on auth/credentials) or custom

Feature will be relatively simple to develop -> just create a configuration and use it.