Add optional Reason field to block dialog

Pitch

I propose adding an option to the "Block User?" confirmation dialog, which allows the user to specify a reason for the block in a text box. If the user chooses to do so, it's appended to the Personal Note on the profile of the blocked user, possibly with a datestamp and/or a "Blocked. Reason:" prefix.

As with the profile notes feature, UI would have to make clear this is only visible to the user themselves and not the blocked account.

As no new functionality is being introduced, the feature should be simple to add.

Motivation

Sometimes I review my blocked accounts to check on whether I have been hasty or overzealous in blocking someone. Also sometimes I come across an account that I've blocked, possibly years ago, and have no idea why. It would be useful to be able to see in the user's profile why I blocked them.

Although this can be achieved by using the existing Personal Note feature, adding an option to the block user process reminds the user that the feature is available, and encourages its use at a point when it's very likely to be useful to them.