displays the number of characters left for the title in the composer

Meta: [Topic length too short](http://meta.discourse.org/t/topic-length-too-short/4707)

This PR modifies the character-left counter in the composer to take into account the restrictions on the title.

![Screenshot_15_03_13_01_06](https://f.cloud.github.com/assets/362783/261807/2e17032e-8d04-11e2-8cb4-e97146c47b56.png)