Add GitHub profile link to Profile, add test for GitHub feature, GitHub profile link only appears on Profile if not blank.
Use method symbol for validation :if option

The simpler if: :method_name syntax can be used for
validates_presence_of because the lambda just calls the method on
the model object.
Reword a double-negative
Merge branch 'small-refactorings' of https://github.com/amar47shah/bridge_troll into amar47shah-small-refactorings