feat: use IMGBB to store images rather than local file system

[sovoid]

Is your feature request related to a problem? Please describe.

Currently, the app uses local file system to store media such as post images. If running on AWS, it stores the images on S3. We can use IMGBB to store images regardless of the platform on which the app is running

Describe the solution you'd like

Modify post handler to store images in IMGBB instead of storing it locally