Implement endpoints to store play session logs

This MR implements the endpoints for storing play
session logs from the game.

Closes #5.

See merge request !3

Setup CI/CD pipeline

This MR configures GitLab CI/CD to run automated tests for every new commits
pushed to the online repository.

Related issue: #3

See merge request !2

Setup the base code using Slim Framework

This MR prepares the initial codebase for the backend
using PHP and Slim Framework.

Closes #1.

See merge request !1