move post-deploy hook

Write $DOKKU_ROOT/$APP/URL before post-deploy hook.
Fix `plugins-install-dependencies` [fixes #720]
Merge pull request #721 from wrboyce/master

Fix `plugins-install-dependencies` [fixes #720]
Let the user know it's on the `stack` target

Since curl is running quietly it may give the impression that the installation hanged with the last message on the terminal being: "sleep 2 # give docker a moment i guess". In my case I thought it gave it a huge moment and not 2 seconds. It wasn't until the third time that I actually checked the processes on the machine and saw there it was curl downloading stuff quietly.
Merge pull request #723 from ghostbar/patch-1

Let the user know it's on the `stack` target
Fixing tabs issue

Sorry
Merge pull request #724 from ghostbar/patch-2

Fixing tabs issue
Merge pull request #664 from alexernst/fix/hostname-url

Don't overwrite $APP/URL modified by plugins in post-deploy hook