MessageList: Avoid entirely replacing html body when possible. #3851
Description
Forking from #3026, which is otherwise mostly resolved.
Currently, the entire html body in the WebView is replaced on any change to the message list (a new message is received, a new emoji reaction is made, a message is edited, etc.). While the largest possible symptom of this, the fresh fetching of images every time this happens, has been remedied by WebView's added ability to cache (see more detail in #3026), it would certainly be nice not to have to do this. EDIT: And, in fact, this doesn't help for realms affected by #3853!
Incremental changes are possible; as @gnprice points out, we don't want to have to maintain diff-checking logic for all possible changes within a message. A good approach is probably to allow the entire replacement of a single message on any change to it. So if it's added, edited, reacted to, starred, etc., treat all these the same way -- just replace that message entirely.
EDIT: Especially with #4527 in mind, it'd be good to allow a message to update separately from its reactions—that issue won't be fixed unless we go further than the "good approach" in that last paragraph.