Create pleroma.email mix task

Closes: https://git.pleroma.social/pleroma/pleroma/issues/1061

lib/pleroma/emails/admin_email.ex

@@ -7,6 +7,7 @@ defmodule Pleroma.Emails.AdminEmail do
 
   import Swoosh.Email
 
+  alias Pleroma.Config
   alias Pleroma.Web.Router.Helpers
 
   defp instance_config, do: Pleroma.Config.get(:instance)
@@ -20,6 +21,19 @@ defmodule Pleroma.Emails.AdminEmail do
     Helpers.feed_url(Pleroma.Web.Endpoint, :feed_redirect, user.id)
   end
 
+  def test_email(mail_to \\ nil) do
+    html_body = """
+    <h3>Instance Test Email</h3>
+    <p>A test email was requested. Hello. :)</p>
+    """
+
+    new()
+    |> to(mail_to || Config.get([:instance, :email]))
+    |> from({instance_name(), instance_notify_email()})
+    |> subject("Instance Test Email")
+    |> html_body(html_body)
+  end
+
   def report(to, reporter, account, statuses, comment) do
     comment_html =
       if comment do