A temporary fix for mention modal #5329

Results : 

![modal_mention](https://cloud.githubusercontent.com/assets/6507951/5229554/8fce6ce6-7719-11e4-9088-5815a4414df7.png)
![modal_mention2](https://cloud.githubusercontent.com/assets/6507951/5229556/92582e8e-7719-11e4-94b3-5d50c35fdcf4.png)

I wanted to add 
````
#new_status_message_pane .modal {
  padding-bottom: 35px; /* or something else */
}
````

But I wasn't sure (little bit ugly).
Yeah, it was a bad idea. In fact, I don't have any backdrop with Firefox on Ubuntu, that's why I didn't understand :)
Now, I have troubles with webkit browser... WIP.