[Feat] Add Favicon feature

[Furkan KURT]

Hello,

Django Unfold doesn't have a favicon feature.
We had to override the whole skeleton.html template to add a favicon. (already mentioned in #206)

I have added a site_favicon.html file to the unfold/helpers directory, we can add all favicon types to the website.
Instead of overriding the big skeleton.html, we will override just the site_favicon.html.

I've already created the PR #382.
I'm excited to see you merging it.