refactor omniauth configuration method

This refactoring allows a user to use omniauth providers which do not
use ```app_id``` and ```app_secret``` without needing to change the
devise configuration.
Merge branch 'master' into dont-depend-on-appid-and-appsecret

* master: (238 commits)
  Version 6.5.1
  Fix selectbox when submit MR from fork to origin
  Fix HELP layout
  No need for code tag here.
  Spelling mistake and add links.
  Warn against RVM.
  Remove GitHub mention because we also have a GitLab issue tracker now.
  Replace 6.0-to-6.4.md with 6.0-to-6.5.md
  Add public assets to gitignore
  Version 6.5.0
  Use 6-5 branch in installation docs
  Remove deprecated twitter handle.
  Further explain userPrincipalName settings
  Update from 6.4 to 6.5 guide
  Explain how to use AD userPrincipalName for logins
  More entries to CHANGELOG. Version to rc1
  Rephrase LDAP check script output
  add O'Reilly sponsorship in CHANGELOG
  Fix select2 css for drop above style
  Rename "Website url" labels to "Website"
  ...
Merge pull request #5957 from ninech/dont-depend-on-appid-and-appsecret

refactor omniauth configuration method