[Fix #196] Attestation: join the attestation to the closed mail

Add a upper limit to the attachment size as it could be a problem with Mailjet and receiver
(https://www.mailjet.com/support/what-is-the-size-limit-for-attachments-files-sent-via-mailjet,289.htm)

If the attestation cannot be sent, it is logged in sentry

app/models/attestation.rb

@@ -2,4 +2,10 @@ class Attestation < ApplicationRecord
   belongs_to :dossier
 
   mount_uploader :pdf, AttestationUploader
+
+  MAX_SIZE_EMAILABLE = 2.megabytes
+
+  def emailable?
+    pdf.size <= MAX_SIZE_EMAILABLE
+  end
 end