Disable 'Save changes' button if input is empty, fixes #6351

#### What does this MR do?

This PR deactivates the save button of a note, if the input is empty (similar as creating a note). Additionally, it changes the class of the `Choose File` button to be the size as when creating a note

Why was this MR needed?

Currently it's possible to edit a comment to be empty.

#### What are the relevant issue numbers / Feature requests?

This PR fixes #6351

#### Screenshots  
##### Before

![screenshot 2014-08-04 20 37 27](https://cloud.githubusercontent.com/assets/278301/3801648/806fb516-1c06-11e4-8282-1b3a497334c2.png)

##### After

![screenshot 2014-08-04 20 38 30](https://cloud.githubusercontent.com/assets/278301/3801663/ac22632a-1c06-11e4-9a23-1c8dbfb2cd5d.png)