Shortcut for calling user by atom

93ee6ee deprecates calls like:

```
escalus_stanza:last_activity(bob)
```

which have to be replaced by:

```
BobShortJID = escalus_client:short_jid(Bob),
escalus_stanza:last_activity(BobShortJID)
```

To make test development easier, maybe we could consider this:

```
escalus_stanza:last_activity(bob, Config)
```
