Deleted user avatar doesn't match format of other avatars

## Bug Report

### Current Behavior
When using a helper function, such as `avatar()`, the output should attempt to stay as similar as possible with outputs documented.

Currently, we have 3 different possibilities for avatars:

**Text avatar**
```html
O
```

**Image avatar**
```html

```

**Deleted user avatar**
```html

```

We should attempt to make the deleted user avatar closer to the Text avatar to prevent styling inconsistencies, such as those currently present in the Notifications area, similar to flarum/framework#2905.

### Suggested changes

We could use a FontAwesome icon in place of the text. This can be done with pure CSS, too. Woohoo!

**user-times**

**user-alt-slash**

**user-slash**

**trash-alt**

### Example

```less
span.Avatar:empty::before {
content: "\f235"; // For user-times
color: @control-color;
font-weight: 700;
font-family: 'font awesome 5 free';
}
```