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.

リンクされた StackBlitz の再現、具体的には app/expansion-overview-example.html から始め、長い expansion-panel の説明を使用します。次に Angular Material の expansion-panel の実装を見つけ、説明がオーバーフローしたりタイトルを押しのけたりしなくなっていることを確認します。完成時の動作としては、適切な切り詰め、またはその他の明確な処理にします。

索引モデルが issue の本文から書いたものです。