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
Since the observer config with childList will listen to childNode add/ remove both, I'm not sure if it will affect performance while startup and creating list item with heavy workload(or deleting lots of thread list, but I think it's minor comparing startup).
Since we don't have it another way for deletion now(and we didn't plain it for the future, maybe delete all message will use ```innerHTML = ''``` someday ) How about we leave a comment for delete api about confirming the element's url revoked before deletion?