separate reply button to its own component, add changelog entry

src/components/reply_button/reply_button.js

@@ -0,0 +1,12 @@
+
+const ReplyButton = {
+  name: 'ReplyButton',
+  props: ['status', 'replying'],
+  computed: {
+    loggedIn () {
+      return !!this.$store.state.users.currentUser
+    }
+  }
+}
+
+export default ReplyButton