add discussions

[frnsys]

this issue will be broken into smaller issues, but broadly:

events and concepts can be discussed – not sure if other things should be discussable or not.

so a given discussion has an entity (an event or a concept), and can additionally have a subject, which can be a summary bullet point in an event, or a particular article within an event

(this model may need reconsideration – perhaps there should only be a subject which could also be an event or a concept, thus rendering entity unnecessary)

a discussion also has posts which constitute the actual conversation.

posts have a parent (which can be None if it is the OP), children, the posts which are in reply to this post (which can be [], if there are no children). and then they also have a user, i.e. whoever created it.

[frnsys]

going to forgo this for now and keep things minimal/simple.