Add "dashboard/base.html" which extends "base.html"

This adds more flexibiltiy to override the default base.html while 
keeping the oscar base.html for the dashboard.

This  can be done by overriding "dashboard/base.html" and then extend
"oscar/base.html" instead of "base.html"
Merge pull request #965 from LUKKIEN/feature/dashboard-template

Add "dashboard/base.html" which extends "base.html"