Add the possibility to add apps on groups. (#459)

* Add the option to add apps on groups

* Add migration script.
It automatically associate all apps of an image to the group associated on
to image.

* App populate with groups

* App tests now test if the app1 included on group1, is correctly retrieve,
but not the app2, who isn't included on group1.
Group test now test to add an remove apps from a group.

* Update documentation on ImageController

* eslint update

* Add a column "Number of applications"
and add more visibility to the code

* Auto select all application of an image, when it is selected, with a
warning saying users can access all they want through windows