Use vuetify directly without the creativetim theme

[kinow]

Describe exactly what you would like to see in an upcoming release

We are not using most of what came with the theme. It was useful for prototyping the initial versions of the app. But after the #87 issue and the design sketch work, we have very little dependency on the theme.

Removing the theme will make it easier to update dependencies. When a new version of Vuetify came, we waited until the theme was updated. After dropping the theme, we can update the app as soon as Vuetify has been released - see #204, #276

Additional context

Pull requests welcome!

[kinow]

The task consists in basically going through the theme (https://github.com/creativetimofficial/vuetify-material-dashboard), looking at what we are still using, and checking if we can use vue.js vanilla or vuetify directly.