Add the same notebook should update the notebook instead of duplicate

[pplonski]

Right now if we run twice the following command:

mercury add notebook_1.ipynb

We will have two notebooks as apps in the Mercury. Please do not add the duplicate, just update the current notebook. By update I mean:

• use the old ID
• run nbconvert on the new notebook
• display a message that notebook is updated (not added)

[pplonski]

When user adds new notebook path then a it will be added to the Mercury. In the case of the notebook path that is already in the Mercury the notebook will be updated.