This change standardises the `User-Agent` header that Discourse will send when talking to other sites.
`Discourse.user_agent` is now the authority on what the user agent value should be. For Onebox requests, this changes the user agent from their existing value to match the new value (unless overridden).
For all other requests, `Net::HTTPHeader` is monkey-patched to add a default `User-Agent` header when one hasn't been provided.
This commit patches `Net::HTTP` to reduce the default timeouts of 60
seconds when we are processing a request. There are certain routes in
Discourse which makes external requests and if the proper timeouts are
not set, we risk having the Unicorn master process force restarting the
Unicorn workers once the `30` seconds timeout is reached. This can
potentially become a vector for DoS attacks and this commit is aimed at
reducing the risk here.
