* Move OpenGraph code into its own file
* Move OpenGraph image proxying to app layer
* Move test file code out of api4 package
* MM-11272 Add OpenGraph and image dimension metadata to posts
Previously, mattermost-server would always request with the default
user-agent of Go's net/http package that is `Go-http-client/1.1` or
something similar.
This has several disadvantages, one is that the default user-agent
made it pretty hard to distinguish mattermost requests from other
service requests in a network log for example.
Now a user-agent of the form `mattermost-<current-version>` is set in
the client.
- [x] Added or updated unit tests (required for all new features)
