Change SQL charset for message_db to utf8mb4

Squashed commit of the following:

commit c434c868d111fd091f638216e2dec2685c7ac296
Author: Charlie Smurthwaite <charlie@krystal.uk>
Date:   Thu Oct 1 10:37:17 2020 +0100

    change utf8mb4_bin to utf8mb4_unicode_ci

commit 68031c470b41edd9906e3b10ccbb6d2c71a2c9a7
Merge: 9f277d4 9b98339
Author: Charlie Smurthwaite <charlie@krystal.uk>
Date:   Thu Oct 1 10:29:02 2020 +0100

    Merge remote-tracking branch 'origin/master' into mb4

commit 9f277d4ce2
Author: Petter Helset <petter@helset.eu>
Date:   Thu Oct 26 15:15:16 2017 +0200

    Migrate message database with main database

commit 3be337e265
Author: Petter Helset <petter@helset.eu>
Date:   Thu Oct 26 14:45:26 2017 +0200

    Migrations for existing messagedbs to utf8mb4

commit 3d8d3956fe
Author: Petter Helset <petter@helset.eu>
Date:   Thu Oct 26 13:49:57 2017 +0200

    Change SQL charset for message_db to utf8mb4

lib/tasks/postal.rake

@@ -34,3 +34,7 @@ namespace :postal do
   end
 
 end
+
+Rake::Task['db:migrate'].enhance do
+  Rake::Task['postal:migrate_message_databases'].invoke
+end