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/contentScripts/communityConsole/darkMode.js b/src/contentScripts/communityConsole/darkMode.js
index d10a77c..d357bad 100644
--- a/src/contentScripts/communityConsole/darkMode.js
+++ b/src/contentScripts/communityConsole/darkMode.js
@@ -1,4 +1,4 @@
-import {createExtBadge} from './utils.js';
+import {createExtBadge} from './utils/common.js';
 
 export function injectDarkModeButton(rightControl, previousDarkModeOption) {
   var darkThemeSwitch = document.createElement('material-button');