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.

Comienza con la reproducción vinculada de StackBlitz, específicamente con app/expansion-overview-example.html, usando una descripción larga del expansion-panel. Después, localiza la implementación del expansion-panel de Angular Material y verifica que la descripción ya no se desborde ni desplace el título, con un truncamiento adecuado u otro tratamiento claro como comportamiento final.

Escrito por el modelo de indexación a partir del texto del issue.