fix(browser-ext): add assets for Chrome Web Store page (#310)

Our "Openwhyd + Track" button (a.k.a. browser extension, or bookmarklet) has been [published on Chrome Web Store](https://chrome.google.com/webstore/detail/foohaghobcolamikniehcnnijdjehfjk/publish-review?hl=fr) for years, making it easy to install for users of Google Chrome.

Recently, Google asked us to update the public page of our extension, but we failed to get our changes accepted. We're not sure yet if this is caused by the wording of its description, or by the choice of images we uploaded to illustrate it.

Here is the admin link: https://chrome.google.com/webstore/devconsole/3db39e57-ca94-4dbb-933b-d9fb843cd022/foohaghobcolamikniehcnnijdjehfjk/edit/listing?hl=fr

In this PR, we add the assets used on our Chrome Web Store page, to keep track of those assets and find them when we need to make updates again.

.dockerignore

@@ -19,3 +19,4 @@
 /config/apns
 /scripts/mongodb-scripts/db
 /scripts/mongodb-scripts/*.gz
+/misc