Add website icon in CMS menu to link to preview of site (in CMS)

[clarkepaul]

I'm proposing we introduce an icon next to the site name in the CMS, developers then could substitute it with a brand icon to align with website name. This icon would also replace the SilverStripe icon used in the collapsed menu at the top. The icon would link users directly to the full preview mode of the home page so the user remains in the CMS (collapsed side menu). It's a really weird experience that the SilverStripe logo opens the site preview outside of the CMS experience (its something that you learn over time but for new users they would never imagine it offers that functionality.

Benefits from change:

• easier to recognise where the icon link leads to
• the icon can be replaced with the site logo and still makes sense with where it links to
• less confusion about how the SilverStripe logo is used as it's also used to link to user help
• for the Sub-sites module, the site icon and site name can easily be replaced with the dropdown selector (potential enhancement which could happen after this issue is done).

Related #783

As per designs: https://projects.invisionapp.com/dsm/silver-stripe/silver-stripe/asset/components/5abad8177894c30011987d2f

[brynwhyman]

Big +1 from me!