Making conversations archivable

Conversation model are now able to be archived with
```@conversation.archive``` should be called on _send & archive_ call.

Conversations can be unarchived with ```@conversation.unarchive```
should be called on incoming email or when a user "opens" a
conversation.

Conversations defaults to only show _unarchived_ conversations for easier listing under list page (archived conversations can be found using ```Conversation.archived```).

For WIP https://assemblymade.com/support-foo/wips/64