Gravatar Image - Adding gravatar image to the Katello User Page

1.Defined some methods for pulling gravatar image for the signed in user and
  displayed it on the Katello User Page and modified the header css.

2.Added a config option for users to enable and disable gravatar on
  Katello User Page.

3.Added a method for testing gravatar enable/disable option and
  also defined gravatar as boolean.
Merge pull request #2497 from pgodhani/gravatar_katello

Gravatar Image - Adding gravatar image to the Katello User Page