Convert send_grid_adapter to SendGrid API v3

[randycoulman]

The existing SendGridAdapter uses v2 of the SendGrid API. In https://sendgrid.com/docs/API_Reference/Web_API/mail.html, it recommends migrating to the new v3 API:

To current v2 users: We encourage migrating to the new endpoint to take advantage of upcoming features.

I was looking at adding attachment support to the existing adapter, and found that it would be much easier to do with the v3 API instead. So, I'm submitting this PR first, and will then add attachment support on top of it.

For reference, the v3 API docs are at https://sendgrid.com/docs/API_Reference/api_v3.html.

I've done my best to preserve the existing API for both the adapter and the helper, but I've changed the internal representation of the private data that the helper adds to the e-mail. The new format is simpler and less coupled to the resulting API format:

%{
  send_grid_template: %{
    template_id: "YOUR ID HERE",
    substitutions: %{
      "%foo%" => "bar"
    }
  }
}

I've tested this adapter in an existing application and it works perfectly, though that app doesn't use all of the available features.

I'm still relatively new to Elixir, so any suggestions for cleaning up the code or making it more idiomatic are most welcome.

[randycoulman]

I just rebased this on the latest master.

[javierjulio]

@randycoulman thanks for doing this! I've been using this in a small Elixir app in production and it works great. Its been very reliable without issue. Let me know if I can be of any help in continuing to test this PR as it gets reviewed.

[devshane]

Hi, any update on merging support for v3? I wonder if this is the cause of a mysterious "Unexpected error" mentioned in #280.

[Schultzer]

@paulcsmith what is the status of this and #294?

[paulcsmith]

@randycoulman Could you rebase this? Once that's done I'll go ahead and merge this in!

[randycoulman]

@paulcsmith Done

[paulcsmith]

Thanks