Module Installer UI Improvements

• add "retry" button for failed install tasks
• add "cancel all" button to clear the task list
• progress bar no longer grows beyond 100%
• improved data rate display (no longer sticks at zero when transferring lots of small files)
• added message about multi-select feature to install page
• update for new upcoming pi image: new root path (/var/www/rachel) and use sudo when calling scripts from new non-root webserver (lighttpd)