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.

Commencez par la reproduction StackBlitz liée, plus précisément par app/expansion-overview-example.html, en utilisant une longue description de l’expansion-panel. Localisez ensuite l’implémentation de l’expansion-panel de Angular Material et vérifiez que la description ne déborde plus et ne repousse plus le titre, avec une troncature élégante ou un autre traitement clair comme comportement final.

Rédigé par le modèle d'indexation à partir du texte de l'issue.