Changes at "Admin contact form"

**Is your feature request related to a problem? Please describe.**

As user I might want to contact admins for support. But most of the time the email is to be found on help page inside a huge text (ex : \"Terms and conditions\") which is hard to find.

**Describe the solution you'd like**

Add a contact form on the main nav or on the home page.

The contact form would have the following fields : name, email, message

The contact form is accessible to unregistered users.

Global admins receive a email notification when the formed is filled and sent by a user.

**Describe alternatives you've considered**

Alternatively we could add a link to create a conversation with an admin but that supposed I was able to create an account (most request we get are people who find it difficult to register).

**Additional context**

There is an existing issue in Github on this topic : https://github.com/decidim/decidim.org/issues/146

Does this issue could impact on users private data?**

No.

**Funded by**

Ville de Toulouse might fund this.