Add changelog failures with Danger.

Dangerfile.swift

@@ -84,3 +84,11 @@ let hasPngs = !editedFiles.filter { $0.lowercased().contains(".xcassets") && $0.
 if hasPngs {
     warn("You seem to have made changes to some resource images. Please consider using an SVG or PDF.")
 }
+
+if danger.github.pullRequest.title.hasSuffix("…") {
+    fail("Please provide a complete title that can be used as a changelog entry.")
+}
+
+if danger.github.issue.labels.filter({ $0.hasPrefix("pr-") }).count != 1 {
+    fail("Please add a pr- label to categorise the changelog entry.")
+}