Fix CORS errors, Set up MySQL connection, New Backend Dockerfile

[yunusemreozdemir]

Issue Description

Since we have migrated to the new backend, we need to fix CORS errors, set up MySQL connection and create the new Dockerfile for backend.

Tasks

• [x] Fix CORS errors
• [x] Set up MySQL connection
• [x] Create backend Dockerfile and requirements.txt

Reviewer Assignment

Reviewer: @Elifnurdeniz