[AddonsLab] Conversation Permissions

[TigerDao]

The add-on adds two new usergroup permission:

1. Create Conversation: Min Post Count
2. Reply to Conversation: Min Post Count

If a user does not have enough posts to start a conversation, they see the "no permission" error when they try to start a conversation.

If a user does not have enough posts to reply to a conversation, they just do not see the reply field.