Option to Auto-Hide Comments

[Alexhuszagh]

For some builds with relatively complex cross-platform checks, it can be difficult to test all the changes locally, resulting in numerous comments that become outdated almost immediately. To simplify viewing the conversation history, it can be helpful to mark most of these comments as outdated. A bors command such as bors comments hide or similar would be quite nice, to allow users to auto-hide most older, outdated tries.