Add filtering conversations by open/closed to the project conversation list

# Problem

The conversation list for projects needs to have an `open/closed` filter placed on it.

We should show a (styled) `select` that has two items:

- Open (3)
- Closed (18)

Where the number in parens is the number of currently open or closed conversations.

This is blocked by implementing a counter cache for open and closed conversations on the API.

沒有指定檔案、測試或進入點。先找出專案的對話清單，並檢查用於開啟和關閉對話計數器快取的 API 工作。完成標準是：一個具樣式的 select 顯示「開啟」和「關閉」及目前的計數，並據此篩選清單。

由索引模型根據 Issue 內容生成。