Make installation workflow more robust

[danielhollas]

The brittleness of the installation workflow partly comes from the fact that the app is installed before the dependencies. That is not common. Pip and conda do exactly the opposite. They first install the dependencies, and only then proceed with installing the main package.

I am thinking that reverting the order might solve a lot of things for us too.

We need to pull the folder to a temporary location (already implemented) and install dependencies(already implemented). If that was successful - copy the folder to ~/apps. It is really about reshuffling the code a tiny bit.

Originally posted by @yakutovicha in #349 (comment)

[danielhollas]

Just to illustrate the importance of this, I was able to trigger the installation of an App twice by clicking the Install button twice in rapid succession.