refactor(extra-info): split code into several classes

This commit refactors the extra info feature so the code is more
maintainable, in preparation for a future commit which will make it work
with the RCE thread page.

It doesn't refactor the code related to the thread view since it will be
heavily modified, and the code related to canned responses has been
deleted since the number of uses of a CR is no longer relevant because
it is no longer counted.

Bug: twpowertools:93
Change-Id: I06c045fb9ff0c824c99f63acfa10976b2110e5ed
diff --git a/src/common/commonUtils.js b/src/common/commonUtils.js
index 5784da1..3e0be66 100644
--- a/src/common/commonUtils.js
+++ b/src/common/commonUtils.js
@@ -32,3 +32,12 @@
   }
   return undefined;
 }
+
+/**
+ * Utility to indicate that a class method should be implemented (similarly to
+ * abstract methods in Java).
+ */
+export function shouldImplement(name) {
+  throw new Error(
+      `The ${name} method should be implemented by the extending class.`);
+}