Store OAuth2 session data in database (#3660)

* Store OAuth2 session data in database

* Rename table to `oauth2_session` and do not skip xormstorage initialization error

vendor/github.com/lafriks/xormstore/README.md

@@ -0,0 +1,48 @@
+[![GoDoc](https://godoc.org/github.com/lafriks/xormstore?status.svg)](https://godoc.org/github.com/lafriks/xormstore)
+[![Build Status](https://travis-ci.org/lafriks/xormstore.svg?branch=master)](https://travis-ci.org/lafriks/xormstore)
+[![codecov](https://codecov.io/gh/lafriks/xormstore/branch/master/graph/badge.svg)](https://codecov.io/gh/lafriks/xormstore)
+
+#### XORM backend for gorilla sessions
+
+    go get github.com/lafriks/xormstore
+
+#### Example
+
+```go
+// initialize and setup cleanup
+store := xormstore.New(engine, []byte("secret"))
+// db cleanup every hour
+// close quit channel to stop cleanup
+quit := make(chan struct{})
+go store.PeriodicCleanup(1*time.Hour, quit)
+```
+
+```go
+// in HTTP handler
+func handlerFunc(w http.ResponseWriter, r *http.Request) {
+  session, err := store.Get(r, "session")
+  session.Values["user_id"] = 123
+  store.Save(r, w, session)
+  http.Error(w, "", http.StatusOK)
+}
+```
+
+For more details see [xormstore godoc documentation](https://godoc.org/github.com/lafriks/xormstore).
+
+#### Testing
+
+Just sqlite3 tests:
+
+    go test
+
+All databases using docker:
+
+    ./test
+
+If docker is not local (docker-machine etc):
+
+    DOCKER_IP=$(docker-machine ip dev) ./test
+
+#### License
+
+xormstore is licensed under the MIT license. See [LICENSE](LICENSE) for the full license text.