Added Contest Mode lesson URL constant

Signed-off-by: Shivam Dixit <shivamd001@gmail.com>
Updated contest challenge model

Signed-off-by: Shivam Dixit <shivamd001@gmail.com>
Added function to get active contest id in contest model

Signed-off-by: Shivam Dixit <shivamd001@gmail.com>
Display the list of challenges on contest home

Signed-off-by: Shivam Dixit <shivamd001@gmail.com>
Added catch controller for contest challenges

Signed-off-by: Shivam Dixit <shivamd001@gmail.com>