danger: relax rules for single-line commits

The following are not also accepted as single-line commits without
generating warnings:
- CHANGES/release note may appear in the beginning of the commit message
- Release note may be capitalized
- Allow commits with "GL #" (e.g. Update documentation for [GL #XXXX])

(cherry picked from commit 79ae467973)

dangerfile.py

@@ -125,8 +125,9 @@ for commit in danger.git.commits:
     if (
         len(message_lines) < 3
         and "fixup! " not in subject
-        and " CHANGES " not in subject
-        and " release note" not in subject
+        and "CHANGES " not in subject
+        and "release note" not in subject.lower()
+        and "GL #" not in subject
     ):
         warn(f"Please write a log message for commit {commit.sha}.")
     for line in message_lines[2:]: