Additional software: fix installation of packages requiring manual confirmation

Originally created by Tails on #6038 (Redmine)

Currently this feature fails to install packages that require manual confirmation. For example sun-java6-jdk. Setting debconf priority to critical should help.

Feature Branch: feature/6038-debconf-priority

Parent Task: #14568 (closed)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information