Rule manager: "Remove all" button should rather be "Mark all for deletion"

[gorhill]

This way, a user can mark all for deletion and, for example, un-mark one scope then commit all. It's tedious to delete all except one.

The warning popup would show only if the amount of rules to delete is, say, above 80% or something like that.

[gorhill]

Fixed in a6a9dcf.