Replace basic auth with cookie authentication (#440)

* Add logout functionality and button
* Make session age configurable

vendor/github.com/gorilla/sessions/options.go

@@ -0,0 +1,18 @@
+// +build !go1.11
+
+package sessions
+
+// Options stores configuration for a session or session store.
+//
+// Fields are a subset of http.Cookie fields.
+type Options struct {
+	Path   string
+	Domain string
+	// MaxAge=0 means no Max-Age attribute specified and the cookie will be
+	// deleted after the browser session ends.
+	// MaxAge<0 means delete cookie immediately.
+	// MaxAge>0 means Max-Age attribute present and given in seconds.
+	MaxAge   int
+	Secure   bool
+	HttpOnly bool
+}