search.php: Add a $content array

Issue #50.
_navbar.html: Add support for user session

Issue #50.
Update uml_template Class diagram
init.php: Fix alignment
template.php: Check file exists before privacy check
Rename 'twig' to 'Twig'
Add a session.php file

Issue #50.
login.php: Name input fields

Issue #50.
login.php: Set the form method and action

Issue #50.
session.php: Refactor for cleanliness

Issues #50 and #93.
template.php: Prefix functions with 'pip_'

Issue #93.
template.php: Annotate existing functions
template.php: Add pip_template_is_private()
template.php: Refactor for cleanliness
session.php: Add a pip_append_session_to_array()
session.php: Add some minor comments