Long expansion panel descriptions overflow the panel.

#### Bug, feature request, or proposal:
If you have a longer than normal expansion panel description it overflows over the expansion panel and pushes the label out of place.

#### What is the expected behavior?
The description should truncate and add an ellipses, or some other way to gracefully hand this situation.

#### What is the current behavior?
If a description is long it will push the title out of place and overflow.

#### What are the steps to reproduce?
https://stackblitz.com/edit/angular-fcxlsc?file=app/expansion-overview-example.html

#### What is the use-case or motivation for changing an existing behavior?
It looks pretty bad if a long description is used.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Tested on Angular Material 5.2.4 using chrome 65.0.3325.181 and firefox 59.0.2.

#### Is there anything else we should know?
Nothing.

Inizia con la riproduzione collegata di StackBlitz, nello specifico app/expansion-overview-example.html, usando una descrizione lunga dell’expansion-panel. Quindi individua l’implementazione dell’expansion-panel di Angular Material e verifica che la descrizione non trabocchi più né sposti il titolo, con un troncamento appropriato o un altro trattamento chiaro come comportamento finale.

Scritto dal modello di indicizzazione a partire dal testo della issue.