Add Minimal Working Code for Frontend and Backend

[hehao98]

Description

GFI-Bot needs to implement:

1. A Python backend using Flask for exposing JSON-based APIs. We choose Flask because it is lightweight and easy to use. This backend will be used to provide information including registered repositories, collected issues, PRs, commits, and users, data collection progress, training progress, the performance of current models, recommendations, etc.
2. A JavaScript frontend using React and React Bootstrap for showcasing our project, attracting potential users, and providing a dashboard for registered users.

As the first step, we need minimal working code for both frontend and backend.

For the frontend, we expect to have a basic react project showing a Home page, a navigation bar, and some copyright and about notices at the tail of each page. We also need a "Repositories" page to list currently registered repositories (in the gfibot.repos collection) and display basic statistics for those repositories. Since the number of repositories may become large, this page needs pagination. For the Home page, we need basic information about GFI-Bot and a three-column description like that on this page. No need to fill in text on the Home page. we will try to fill them later.

For the backend, we expect to have some APIs to return currently registered repositories as a paginated list.

Files to Touch

The main file changes should be made in the frontend folder and the gfibot/backend folder. You can also add tests, new dependencies, and new GitHub workflows (.github/workflows) where necessary.