Preserve hashtag casing in threaded mode

Inspired by f9b23a5d62

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

@@ -0,0 +1,8 @@
+export function recoverHashtags (recognizedTags, text) {
+  return recognizedTags.map(tag => {
+      const re = new RegExp(`(?:^|[^\/\)\w])#(${tag.name})`, 'i');
+      const matched_hashtag = text.match(re);
+      return matched_hashtag ? matched_hashtag[1] : tag;
+    }
+  );
+}