Install hangs at theme selection task

Install is hanging after submitting the theme selection form task.

Traced the issue to conditional adding of Subprofiles tasks.

• Was testing for module_exists('subprofiles') before adding these tasks.
• Before a full bootstrap, this test returns FALSE. When the completed_task is one from subprofiles, it's never found and so all subsequent tasks are skipped. See install_run_tasks(). As all tasks after bootstrap are skipped, the installer hangs.

Fix: explicitly load the subprofiles include file so it's always available.