* initial stuff for oauth2 login, fails on:
* login button on the signIn page to start the OAuth2 flow and a callback for each provider
Only GitHub is implemented for now
* show login button only when the OAuth2 consumer is configured (and activated)
* create macaron group for oauth2 urls
* prevent net/http in modules (other then oauth2)
* use a new data sessions oauth2 folder for storing the oauth2 session data
* add missing 2FA when this is enabled on the user
* add password option for OAuth2 user , for use with git over http and login to the GUI
* add tip for registering a GitHub OAuth application
* at startup of Gitea register all configured providers and also on adding/deleting of new providers
* custom handling of errors in oauth2 request init + show better tip
* add ExternalLoginUser model and migration script to add it to database
* link a external account to an existing account (still need to handle wrong login and signup) and remove if user is removed
* remove the linked external account from the user his settings
* if user is unknown we allow him to register a new account or link it to some existing account
* sign up with button on signin page (als change OAuth2Provider structure so we can store basic stuff about providers)
* from gorilla/sessions docs:
"Important Note: If you aren't using gorilla/mux, you need to wrap your handlers with context.ClearHandler as or else you will leak memory!"
(we're using gorilla/sessions for storing oauth2 sessions)
* use updated goth lib that now supports getting the OAuth2 user if the AccessToken is still valid instead of re-authenticating (prevent flooding the OAuth2 provider)
* Correction LDAP username validation
As https://msdn.microsoft.com/en-us/library/aa366101(v=vs.85).aspx describe spaces should not be in start or at the end of username but they can be inside the username. So please check my solution for it.
* Check for zero length passwords in LDAP module.
According to https://tools.ietf.org/search/rfc4513#section-5.1.2 LDAP client should always check before bind whether a password is an empty value. There are at least one LDAP implementation which does not return error if you try to bind with DN set and empty password - AD.
* Clearing the login/email spaces at the [start/end]
* Write LDAP, SMTP, PAM, DLDAP back to all uppercase
* Fix type in unused constant name
* Other MixCased fixes
* Complete MixerCasing of template constants
* Re uppercase LTS and LDAPS suffixes
* Uppercase JSON suffix in constant names
* Proper case LoginNoType
* Prefix unexported template path constants with "tpl"
