Add a theme to the app

[LucileDT]

Context

To make the app a bit better to look at and easier to use, we should install a theme.

We decided to use NiceAdmin for now.

Todo

• [x] install Bootstrap
• [x] install NiceAdmin
• [x] create a navbar and put connected links & user info in it
• [x] update homepage to make it a default landing

[LucileDT]

@Yann-BUTSCHER-EIRL please test it when you have some time 🙂