Add option to not automatically unfold content warnings matching a regexp

Fixes #678

app/javascript/flavours/glitch/util/content_warning.js

@@ -0,0 +1,19 @@
+export function autoUnfoldCW (settings, status) {
+  if (!settings.getIn(['content_warnings', 'auto_unfold'])) {
+    return false;
+  }
+
+  const rawRegex = settings.getIn(['content_warnings', 'filter']);
+  let regex      = null;
+
+  try {
+    regex = rawRegex && new RegExp(rawRegex.trim(), 'i');
+  } catch (e) {
+    // Bad regex, don't affect filters
+  }
+
+  if (!(status && regex)) {
+    return undefined;
+  }
+  return !regex.test(status.get('spoiler_text'));
+}