Bug 994553 - [Sora][Message][Contacts]Contacts's head portrait do not display in Message

- Store the photo url to li dataset and revoke url when element removed from DOM tree
I'm wondering if we should not use a MutationObserver on the `<ul>` element instead ? see https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver#MutationRecord we can access deleted notes in the `removedNodes` property.

I'm afraid of having several ways of removing a thread, even if right now we have only one.
One example is in the unit test: we just call `innerHTML = ''`, even if it's not really a concern.

So, what do you think?