Convert Add-Ons control panel into a browser view

This has already been mostly done by @vangheem in the plip-13679-auto-csrf branch of plone.app.controlpanel.  We need to:

1. Move the views to Products.CMFPlone.controlpanel.
2. Make sure there are tests.
3. Remove obsolete files from the plone_prefs skin layer:

* prefs_install_product_readme.pt
* prefs_install_product_readme.pt.metadata
* prefs_install_products_form.pt
* prefs_install_products_form.pt.metadata
* prefs_installed_product_overview.pt
* prefs_installed_product_overview.pt.metadata
* prefs_reinstallProducts.py