branding: Allow templates to be replaced

- Allowed themes the ability to override the default behaviour of
  append their templates to previous template. This allows themes
  to almost completely replace the entire welcome page.

Change-Id: Id3376500d1ceece80132089d90174e092992d36d
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1055625
Signed-off-by: Alexander Wels <awels@redhat.com>