Bug 1078384 - [MMS] Move MMS label to appear always at the end of the subject line

> mmm it's flex element, why do we need ```calc```? Can we just use the following snippet?:
```
flex: 1;
min-width: 0; // see https://bugzilla.mozilla.org/show_bug.cgi?id=1043520 for details
```

Is my advice from previous review doesn't work for you for some reason?

Also I think we don't need ```display: flex``` here anymore