use "staged" instead of "installed" for Caskroom

refs: #6783
demote the Tap migration banner in FAQ.md
hide error outputs from plutil
Add Stepmania.app v5beta4
basic docs for stub gpg stanza
Merge pull request #6866 from jordannoble/stepmania

Add Stepmania.app v5beta4a
convert adobe-air.rb to use `installer :script`

* remove `caskroom_only true`
* remove `postflight`
convert amazon-music.rb to use `installer :script`

* remove `caskroom_only true`
* remove `postflight`
use `installer :script` in private-internet-access

* hoist primary command out of `postflight` into `installer` artifact
* this Cask did not use the `caskroom_only` idiom
use installer :manual in adobe-creative-cloud
use installer :manual in backblaze
use installer :manual in baiduinput
use installer :manual in dbvisualizer
use installer :manual in default-folder-x
use installer :manual in google-nik-collection
use installer :manual in jdownloader
use installer :manual in little-snitch
use installer :manual in logmein-hamachi
use installer :manual in paye-tools
use installer :manual in ps3-media-server