First Cypress integration test: user login.

Currently, in admin panel, we have installed Cypress and have some experimental tests under client/cypress/integration, now we need to put them together to work correctly, requirements:

* A new command under server folder
```
cd server
npm run start:integration
```
to run a special server for the integration tests only. The major difference from the normal server is that it would seed data for test automatically.

* Seed basic data into the DB

Currently, just need minimal data for user login. Like: a user record in 'amdin_user', and some necessary data like: admin roles. With these data, we can emulate user login in the first integration test.

* Cypress test: login

Build a test to login using the data seeded in DB by the step above. (We already have some basic setting for Cypress and example tests under client/cypress/integration folder)

So in this way, we can run the login test and all the data is self-sufficient, we just need a connection to connect to the database.