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
I can change it to 100, I did use 20 because that is what we use elsewhere but I do agree that 20 is low.