Flash/Hidden CTP

Land and uplift the following to 50:

• bug 1289802 - replace "install flash" content with click to play UI when flash is click to play
• bug 1294341 - add a dummy plugin for sites that check navigator.plugins.length
• switch pref to be a list of plugins to hide
  • will put the patch on bug 1294341 to accommodate a test for that patch)
  • initial state will be "Shockwave Flash" for Nightly and unset for other channels
• bug 1298083 - don't hide block listed plugins

In the back pocket:

• If needed, we can test display the c2p icon if the dummy plugin is touched