Add client side opt in for image inlining.

Refers to #92.

Currently to preserve behaviour image inling is turned on by default and
the user has no option to disable it. This will be added later.
Extract methods for navigation sections.

This allows adding new sections which contains other items than simple
links.
Add preference section to navigation box.

Fixes #92.

This enables the user to opt in for image inlining. By default image
inlining is disabled.