UX explaining inst dash email subject length (128 characters)

@marcotuts or @talbs mind doing a quick design review? Should take 2 minutes.

Addresses https://edx-wiki.atlassian.net/browse/LMS-1187

Limits the length of the input box to 128 characters (the backend model has this restriction) and adds explanatory text.

*Beta dash screenshot:*
![screen shot 2013-10-23 at 11 30 54 am](https://f.cloud.github.com/assets/1985317/1391165/86828d28-3bf8-11e3-9a6b-d2d167bbb268.png)

*Legacy dash screenshot*
![screen shot 2013-10-23 at 11 29 22 am](https://f.cloud.github.com/assets/1985317/1391166/8682830a-3bf8-11e3-89c2-200e80c0f1c4.png)

@flowerhack take a quick look as well, thanks.