WIP 129 message api fixes

Per: https://assemblymade.com/helpful/wips/129

#### Api::MessagesController
- Create a new conversation if no conversation_id is present
- Require the account parameter which should be the slug of the account on which to create the conversation
- Add Tests

#### embed.js
- Fix remaining references to Supportly -> Helpful
- Support the new account parameter and update comments to reflect this

#### Other Changes
- Add accepts_nested_attributes_for :conversation to Messages
- Require :account on Conversation
- Use FactoryGirl in conversation_test