Improve mobile usability

This PR improve mobile version:
It's easier to click on the link to compose a new message
**Before**
![compose-before](https://f.cloud.github.com/assets/930064/916735/ced27a66-fe95-11e2-9e15-4c27fd8df868.png)
**After**
![compose-after](https://f.cloud.github.com/assets/930064/916736/d06bfbea-fe95-11e2-8e8f-497dd3470f28.png)

The publish button and the upload photo button is now at 220px instead of 300px from the top: they are not masked by the mobile keyboard anymore.
![buttons-after](https://f.cloud.github.com/assets/930064/916740/ef157526-fe95-11e2-9f87-51c54076d033.png)