Remove disabled navigation in ISIS

#### Small UX improvement in ISIS template

##### What is it?
On ISIS when we are editing something (basically when we are in singular view) the main navigation becomes greyed out, unavailable but still visible. This PR hide it altogether. If you cannot click it don’t show it!

#### Tests
Apply the patch and move around in the admin area

##### In pictures:
Normal view
![screenshot 2014-10-16 13 34 35](https://cloud.githubusercontent.com/assets/3889375/4661252/56b0838c-5521-11e4-8ec0-545311a61367.png)

Enter editing mode
![screenshot 2014-10-16 13 34 21](https://cloud.githubusercontent.com/assets/3889375/4661254/5b831ec4-5521-11e4-84d4-5dba98de46a0.png)

Scrolling on editing mode
![screenshot 2014-10-16 13 34 11](https://cloud.githubusercontent.com/assets/3889375/4661260/6d587b30-5521-11e4-8ae2-6d6f9fea6b42.png)