Dangerfile: warn when MR needs tests, thank when updating docs

dangerfile.ts

@@ -1,15 +1,13 @@
-import { danger, warn } from 'danger';
+import { danger, warn, message } from 'danger';
 
-// No PR is too small to include a description of why you made a change
-if (danger.gitlab.mr.description.length < 10) {
-  warn('Please include a description of your PR changes.');
+const docs = danger.git.fileMatch('docs/**/*.md');
+const app = danger.git.fileMatch('app/**/*.(js|ts|tsx)');
+const tests = danger.git.fileMatch('*/__tests__/*');
+
+if (docs.edited) {
+  message('Thanks - We :heart: our [documentarians](http://www.writethedocs.org/)!');
 }
 
-// Check for a CHANGELOG entry
-const hasChangelog = danger.git.modified_files.some(f => f === 'CHANGELOG.md');
-const description = danger.gitlab.mr.description + danger.gitlab.mr.title;
-const isTrivial = description.includes('#trivial');
-
-if (!hasChangelog && !isTrivial) {
-  warn('Please add a changelog entry for your changes.');
+if (app.modified && !tests.modified) {
+  warn('You have app changes without tests.');
 }