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

[joshsmith]

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.