Add featuredSessions database rule
Add featuredSessions database rule

Without this rule, users will not be able to save their featured sessions in
Firebase.

Fixes #309

Signed-off-by: Dan Scott <bb18028a90036a9730455495bac08c039a6bcf59@gmail.com>

GDGYangon/gdg-yangon-devfest-2015-website,GDGYangon/gdg-yangon-devfest-2015-website