Add plug to do board/user lookups and needed error responses.

[zorn]

Currently the JoinBoard.process_request/3 takes a user_id and a board_id as part of it's argument list. I keep flip flopping about this, if I should pass in an id or the User/Board.

My current thinking is that, the controller function should be more explicit, probably through a plug about doing lookups for the board and user. If the request is referencing a non-existent resource, then we should return a HTTP error code, showing an un-proccessable request -- this way the internal functions can have a tighter scope of responsibilities, and not have to constantly check for bad references to non-existent Users.

[zorn]

Right now JoinBoard.process_request/2 is called after an attempt to call Boards.get_board!(board_id) so I feel confident that if the board_id is not valid a 404 response will be sent.

Similarly we attempt to get user_id via user_id = Plug.Conn.get_session(conn, :user_id) -- the expectation is this will result in a nil value OR valid user_id. Technically it could result in an invalid user_id, I see this happen when I reset my local dev db but the browser cookie still has the old user id but I don't see this happening much once in production.

Having JoinBoard.process_request/3 return {:ok, user, board} or {:error, reason} just keeps the code simple. I suppose in a more bullet proof env I could return :board_not_found or :user_not_found but I'm going to skip that for now.

If we did add this in the future, I suppose the good user experience would be to show a 404 for missing board, or forward the user to a help page explaining they should reset their local cookies and try again.