Improve visualization of deleted messages

[emilylange]

Currently, deleted messages are visualized with a rather meaningless bar:

While there is this nice tooltip when hovering above, it isn't really intuitive.
I thought about two ways:

• explanation what this is when a new user sees it the first time
  or
• replace the bar with an actual text like pattle.im does:

[yannicka]

Here's what it looks like now on Riot Web/Desktop 1.6.2:

[t3chguy]

Ah yes, this had a duplicate issue which I closed but didn't know about this one

[emilylange]

Awesome 👍
Good job 🚀