Don't change dashboard on admin tenant switch

Switching projects as an admin user currently dumps you back to
the Admin dashboard tab. This change fixes that by adding a 'next'
parameter to the switching URL that points to the default panel
of the current dashboard.

The redirect is to the default panel rather than the current one,
since the next tenant may not have access to the current panel
and we want to avoid kicking a user back to the login screen.

Fixes bug #1072848.

Change-Id: I25b8c0e38945d5db57f5bfe7bb174cdbd66156d5
Merge "Don't change dashboard on admin tenant switch"