Move all serializers into a single file in the same dir

[musale]

Description

Move serializers in the interface app into a single file for each directory

Reference to official issue

Addresses issue #201

Motivation and Context

Makes the app structure cleaner and concise

How Has This Been Tested?

interface.backend.cases and interface.backend.images have a serializers.py file that holds all the respective serializers previously situated in their models.py files.

CLA

• [X] I have signed the CLA; if other committers are in the commit history, they have signed the CLA as well

[lamby]

LGTM, just seeing some conflicts... :)

[musale]

@lamby I have fixed the conflicts now 😄