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 2014-08-11 09 54 57](https://cloud.githubusercontent.com/assets/278301/3873233/e61d1c52-212c-11e4-92e8-ed6dae7c89ab.png)

##### After

![screenshot 2014-08-11 09 58 03](https://cloud.githubusercontent.com/assets/278301/3873261/3e6c1138-212d-11e4-935c-4ac95f7022e2.png)

@jvanbaarsen Can you take a look?