* add google+

* sort signin oauth2 providers based on the name so order is always the same

* update auth tip for google+

* add gitlab provider

* add bitbucket provider (and some go fmt)

* add twitter provider

* add facebook provider

* add dropbox provider

* add openid connect provider incl. new format of tips section in "Add New Source"

* lower the amount of disk storage for each session to prevent issues while building cross platform (and disk overflow)

* imports according to goimport and code style

* make it possible to set custom urls to gitlab and github provider (only these could have a different host)

* split up oauth2 into multiple files

* small typo in comment

* fix indention

* fix indentation

* fix new line before external import

* fix layout of signin part

* update "broken" dependency

* 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]

- Update import paths from github.com/go-gitea to code.gitea.io
- Fix import path for travis

See https://docs.travis-ci.com/user/languages/go#Go-Import-Path

* 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"

Unix() already uses UTC as timezone

Initially proposed by #2376 and fixes #3068 as well.

Add AttributesInBind option in new auth source form.

Consider following LDAP search query example:

    (&(objectClass=Person)(|(uid=%s)(mail=%s)))

Right now on first login attempt Gogs will use the text supplied on login form
as the newly created user name. In example query above the text matches against
both e-mail or user name. So if user puts the e-mail then the new Gogs user
name will be e-mail which may be undesired.

Using optional user name attribute setting we can explicitly say we want Gogs
user name to be certain LDAP attribute eg. `uid`, so even user will use e-mail
to login 1st time, the new account will receive correct user name.