User dashboard: add colours to challenges and exercies panel.

Should make it easy to distinguish. Using orange for "Welcome" and "Exercies"
as shades or red are supposed to be attention catching and that's what we want
users to see first. Using bootstrap label/btn-primary for "Challenges".

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>
User dashboard: fix typo in exercises panel.

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>