Feature - Add category to conversation statuses

[abhinavxd]

• Since admins can add custom statuses, we need to support status categories.
• For example, an admin might add an On Hold status. Libredesk has no way of knowing whether this means the conversation is Open or the agent is Waiting on contact to respond.
• This distinction is important because conversation auto-assignment respects assignment limits, and should behave differently based on the status category.
• By categorizing statuses, the auto-assignment logic can correctly decide whether to assign new conversations based on the current status category.

Possible categories:

• Open
• Waiting