Add methods to reduce necessary null checks

[davidstanley01]

Across the entities such as Conversation, Thread we should add methods like $conversation->isAssigned() rather than having to do $conversation->getAssigned() === null.

There's a lot of perks here:

• keeps our customer's code cleaner and easier to read, reducing possible errors they'll encounter
• makes the way our application works easier for developers to understand
• boosts developer confidence
• enables us to reduce the number of methods that return mixed values

[davidstanley01]

Partially addressed in #82. I'm going to take another look at things before closing this out as complete.