Add X-Hub-Signature header to webhook deliveries

[BenLubar]

The format is sha1= followed by a hex SHA1-HMAC made in the same way as the current sha256 Gitea generates for its own signature header.

[coolaj86]

@techknowlogick @lunny This is something that I may be able to take on.

It looks like the Signature field is currently pre-computed, but could probably be omitted from structs and storage and computed as-needed from the PayloadContent as to support multiple signature types.