Fix previewing banner on-wiki

Change-Id: I68933ce42015f61f89cce3fbdcc83b3038607da3
default to client banner choice

This is to make QUnit testing more realistic, also cos this
will be the supported default, moving forwards.

Change-Id: Ib5726ff3270e8ffbacfb048467ad6d845a63d080
Merge master to wmf_deploy

055fb9380cd91bdfc14d77f797fa4e70b6deb5d4 Fix previewing banner on-wiki
7721b32b1df7cc141fc6e5ac01fcf773b35d57f5 default to client banner choice

Change-Id: Ib47103e2876bbc7409da6d551859bf42c7d04b2d