link to web app home page from the datalab home page and vice versa #116

Not sure if this is wanted or an oversight, but once we are on the Datalab home page, there is no link to the Web App home page. The user needs to go back to the project web page (link available at the bottom), or know the URL construction. One option would be to place it top right on the left of the user name? The same could be done from the Web App. This may raise awareness of each of them to user, passing from one to another easily, without having to look or type the project home page.

It seems to me quite important thus the sept-release label, but to be validated naturally.