Change gravatar_icon helper to handle ssl

This commit refactors the gravatar_icon helper method to handle ssl.
This is because when running gitlab-ci with ssl enabled, it gives a
warning because portions of the page are not secure. In addition to
refactoring this method, it also adds some configuration values to the
application.yml.example file that are needed for the gravatar_icon
helper method. And finally, this commit adds spec tests to cover the
refactored method.