adds message title, adds sender delete

- message now contains a second string
  which is meant for unencrypted title
  information, which can be displayed
  without e2e encryption.
- senders can now be deleted
- the same sender / token connection is
  no longer added to the database

lib/bump/messages/message.ex

@@ -4,6 +4,7 @@ defmodule Bump.Messages.Message do
 
   schema "messages" do
     field :sender, :string
+    field :title, :string, default: ""
     field :data, :string, default: ""
     field :timestamp, :utc_datetime
     timestamps()