Fixes #7000/BZ1080172: paginate content host packages.

Rendering hundreds of content host package rows was taxing the CPU,
especially in Firefox.  This commit uses client side pagination to
limit the number of package rows rendered.

http://projects.theforeman.org/issues/7000
https://bugzilla.redhat.com/show_bug.cgi?id=1080172
20 seems like a really low limit here to me.  How does something like 100 perform?