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)

templates/user/settings/account_link.tmpl

@@ -0,0 +1,48 @@
+{{template "base/head" .}}
+<div class="user settings account_link">
+	<div class="ui container">
+		<div class="ui grid">
+			{{template "user/settings/navbar" .}}
+			<div class="twelve wide column content">
+				{{template "base/alert" .}}
+				<h4 class="ui top attached header">
+					{{.i18n.Tr "settings.manage_account_links"}}
+				</h4>
+				<div class="ui attached segment">
+					<div class="ui key list">
+						<div class="item">
+							{{.i18n.Tr "settings.manage_account_links_desc"}}
+						</div>
+						{{if .AccountLinks}}
+						{{range $loginSource, $provider := .AccountLinks}}
+							<div class="item ui grid">
+								<div class="column">
+									<strong>{{$provider}}</strong>
+									{{if $loginSource.IsActived}}<span class="text red">{{$.i18n.Tr "settings.active"}}</span>{{end}}
+									<div class="ui right">
+										<button class="ui red tiny button delete-button" data-url="{{$.Link}}" data-id="{{$loginSource.ID}}">
+											{{$.i18n.Tr "settings.delete_key"}}
+										</button>
+									</div>
+								</div>
+							</div>
+						{{end}}
+						{{end}}
+					</div>
+				</div>
+			</div>
+		</div>
+	</div>
+</div>
+
+<div class="ui small basic delete modal">
+	<div class="ui icon header">
+		<i class="trash icon"></i>
+		{{.i18n.Tr "settings.remove_account_link"}}
+	</div>
+	<div class="content">
+		<p>{{.i18n.Tr "settings.remove_account_link_desc"}}</p>
+	</div>
+	{{template "base/delete_modal_actions" .}}
+</div>
+{{template "base/footer" .}}

templates/user/settings/navbar.tmpl

@@ -22,6 +22,9 @@
 		<a class="{{if .PageIsSettingsTwofa}}active{{end}} item" href="{{AppSubUrl}}/user/settings/two_factor">
 			{{.i18n.Tr "settings.twofa"}}
 		</a>
+		<a class="{{if .PageIsSettingsAccountLink}}active{{end}} item" href="{{AppSubUrl}}/user/settings/account_link">
+			{{.i18n.Tr "settings.account_link"}}
+		</a>
 		<a class="{{if .PageIsSettingsDelete}}active{{end}} item" href="{{AppSubUrl}}/user/settings/delete">
 			{{.i18n.Tr "settings.delete"}}
 		</a>

templates/user/settings/password.tmpl

@@ -9,13 +9,15 @@
 					{{.i18n.Tr "settings.change_password"}}
 				</h4>
 				<div class="ui attached segment">
-					{{if .SignedUser.IsLocal}}
+					{{if or (.SignedUser.IsLocal) (.SignedUser.IsOAuth2)}}
 					<form class="ui form" action="{{.Link}}" method="post">
 						{{.CsrfTokenHtml}}
+						{{if .SignedUser.IsPasswordSet}}
 						<div class="required field {{if .Err_OldPassword}}error{{end}}">
 							<label for="old_password">{{.i18n.Tr "settings.old_password"}}</label>
 							<input id="old_password" name="old_password" type="password" autocomplete="off" autofocus required>
 						</div>
+						{{end}}
 						<div class="required field {{if .Err_Password}}error{{end}}">
 							<label for="password">{{.i18n.Tr "settings.new_password"}}</label>
 							<input id="password" name="password" type="password" autocomplete="off" required>