Add button to link out to addons.silverstripe.org

[be2n]

As a Product Owner
I want to find other modules I can use on my site
So that I can reuse code when possible

Acceptance criteria

• There is a link out to the add-on site

Designs: https://projects.invisionapp.com/share/ASGU6A7RT8U#/screens/291050684

[raissanorth]

@clarkepaul @sachajudd Could you please add the icon(s) from the design to Fontastic, please? I am happy to add it(/them) to the admin repo once that's done.

[clarkepaul]

This will be a task for @newleeland, he should be able to sort before next week.