[WIP] Mailview factories

Create up `factories.rb`

Factories should NOT save anything to the database. Never.
This validation was deleted, since it doesn't make much sense for a couple of reasons:

1. Payment belongs to conversation. Thus, conversation is the one that should validate that it has right number of payments (1). So, this should be in conversation.rb
2. The conversation<->payment association is `has_one`, so there will never be more than one, unless someone screws up something and manipulates DB manually.