Moved apps/zadmin/ to mkt/zadmin/ (bug 1021207)

https://bugzilla.mozilla.org/show_bug.cgi?id=1021207

Don't merge yet : Because of broken tests, I removed some old code for managing & searching addons that is actually still used by @eviljeff and al. Need to implement [bug 1013472](https://bugzilla.mozilla.org/show_bug.cgi?id=1013472) first.
A lot of times you can substitue `['base/users']` with `fixture('user_999')` -- or sometimes not even that since the webapp fixture comes with a user.