Voila-vuetify

- Push the value of dark prop to the back-end (54)
- fix: mdi icon version inconsistency with ipyvuetify (55)
- support Voila 0.3 (58)
- fix: wrong boxing model set for Vuetify (56 )

- support for Voila 0.2

* fix: theme config for base and third party template
* feat: make it possible to enable the vuetify dark theme from the CLI
* feat: show widget loading stage in the loader

Features
- Dark or light theme configurable in URL by adding `?theme=dark` or `?theme=light`
- Support for [mdi icons](https://cdn.materialdesignicons.com/4.5.95) (introduced in ipyvuetify 1.2.2)
- Support for ipyvuetify 1.4.0 (based on Vuetify 2.2.26)
- New mount points: `content-title` and `content-bar`

Bug fixes
- Add missing Jupyter (fontawesome) icons

Fixes: 28 and part of 24