Oauth2 consumer (#679)

* 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)

vendor/golang.org/x/oauth2/internal/transport.go

@@ -0,0 +1,69 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package internal contains support packages for oauth2 package.
+package internal
+
+import (
+	"net/http"
+
+	"golang.org/x/net/context"
+)
+
+// HTTPClient is the context key to use with golang.org/x/net/context's
+// WithValue function to associate an *http.Client value with a context.
+var HTTPClient ContextKey
+
+// ContextKey is just an empty struct. It exists so HTTPClient can be
+// an immutable public variable with a unique type. It's immutable
+// because nobody else can create a ContextKey, being unexported.
+type ContextKey struct{}
+
+// ContextClientFunc is a func which tries to return an *http.Client
+// given a Context value. If it returns an error, the search stops
+// with that error.  If it returns (nil, nil), the search continues
+// down the list of registered funcs.
+type ContextClientFunc func(context.Context) (*http.Client, error)
+
+var contextClientFuncs []ContextClientFunc
+
+func RegisterContextClientFunc(fn ContextClientFunc) {
+	contextClientFuncs = append(contextClientFuncs, fn)
+}
+
+func ContextClient(ctx context.Context) (*http.Client, error) {
+	if ctx != nil {
+		if hc, ok := ctx.Value(HTTPClient).(*http.Client); ok {
+			return hc, nil
+		}
+	}
+	for _, fn := range contextClientFuncs {
+		c, err := fn(ctx)
+		if err != nil {
+			return nil, err
+		}
+		if c != nil {
+			return c, nil
+		}
+	}
+	return http.DefaultClient, nil
+}
+
+func ContextTransport(ctx context.Context) http.RoundTripper {
+	hc, err := ContextClient(ctx)
+	// This is a rare error case (somebody using nil on App Engine).
+	if err != nil {
+		return ErrorTransport{err}
+	}
+	return hc.Transport
+}
+
+// ErrorTransport returns the specified error on RoundTrip.
+// This RoundTripper should be used in rare error cases where
+// error handling can be postponed to response handling time.
+type ErrorTransport struct{ Err error }
+
+func (t ErrorTransport) RoundTrip(*http.Request) (*http.Response, error) {
+	return nil, t.Err
+}