Images that are attached to messages, either sent or received
can be opened in a new tab by clicking on them.
The previous approach that used Anchors to open the image
attachmets failed in various systems because:
- Chrome on Windows recognised "blob" as protocol and tried
to find an app for it
- Chromium on Ubuntu didn't open a new window to load the URL
The new approach adds a "click" listener to the IMG element and
opens the link using window.open (which seems to be working globaly).
Resolves: #252
Converting attachment data to base64-encoded data uris takes O(n) and
there's no need! URL.createObjectURL returns a magic link that can be
set as the `src` attribute to `img`, `video`, and `audio` tags to load
blob data directly without copying.
https://developer.mozilla.org/en-US/docs/Web/API/URL/createObjectURL
