Cache thread avatars

This change adds the AvatarsDB class, which lets the threadListAvatars
feature interact with a cache of thread avatars.

This is an implementation of points 1 and 2 in the "Idea" section of the
following doc: go/eu7T9m (public link available in the linked bug). The
doc includes a rationale for this change and what it does.

Bug: 2
Change-Id: Ida9fcd909e3bd4a552361317b9013cb8734272a6
diff --git a/src/common/xhrInterceptors.json5 b/src/common/xhrInterceptors.json5
index 158edb9..cc0699f 100644
--- a/src/common/xhrInterceptors.json5
+++ b/src/common/xhrInterceptors.json5
@@ -1,9 +1,9 @@
 {
   interceptors: [
-    /*{
-      eventName: "ViewThreadResponse",
-      urlRegex: "api/ViewThread",
+    {
+      eventName: "ViewForumResponse",
+      urlRegex: "api/ViewForum",
       intercepts: "response",
-    },*/ // Example
+    },
   ],
 }