* Use a custom user-agent when previewing links
Many websites block requests made with Go's default user-agent. We had
previous special-cased Twitter links to use a nonstandard user-agent.
This makes that behavior apply everywhere and also customizes the
user-agent to belong specifically to Mattermost.
* Correctly use custom transport for link previews
This allows us to use the custom user-agent defined in
services/httpservice/client.go.
* Stop leaking server version in custom user-agent
Since the custom user-agent is now used when previewing links, exposing
the server version could provide a vector for a malicious actor to
gather information about private deployments. To avoid this, we switch
to a generic string.
* Remove extraneous Transport creation
MakeClient already creates a transport for us, so this is unnecessary.
Co-authored-by: Mattermod <mattermod@users.noreply.github.com>
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