[front-end] Modify the site header & the add building page

### Add building page
Modify the site width to match the [WebFactory site](https://acasa.website-factory.heroesof.tech/).

### Header
The only page needing the header is the `add building` form.
Redesign the site header according to the new site design.

The header should only contain the following elements:
- [x] the logo should take you to the WebFactory site
- [ ] `back to the site` call to action to take you back to the site
- [x] the language switcher
- [ ] the partner logos, moved from the footer

Add the `a Commit Global` solution banner as it appears on the WebFactory site.

The links to other pages should disappear since they will appear on the WebFactory site.

Mock of how the header might look like (the _back-to-site_ button is missing):