Some images/icons are currently without an "alt" attribute

**Steps to reproduce:**

```
The sidebar button, "..." button and the search icon are all missing alt attributes.
```

**Expected behavior:**

```
All images should have an alt attribute.
```

**Actual behavior:**

```
Alt attributes are missing for some elements.
```

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-104)