Refactor prod validators to be split into individual validators

Refactor prod_validators.py such that there is one validator module per model module in core/storage/.

List of model categories

TODO

In progress

• subtopic [John Jia]

Done

• activity @ankita240796 (#10416)
• audit @kate-perkins (#10995)
• base_model @ankita240796 (#10360)
• classifier @rlhouser (#11116)
• collection @KhushiGangopadhyay (#11152)
• config @iramin (#11148)
• email @krishita30j (#11077)
• feedback @archi15 (#11340)
• exploration @hardikkat24 (https://github.com/oppia/oppia/pull/11926)
• improvements @MeetVys (#10952)
• job @nskirkey (#11373)
• opportunity @savithakj (#10735)
• question @mozammel (#11222)
• recommendations @sunny1401 (#10855)
• skill @sajalasati (#10907)
• statistics @arora-ria (#10960)
• story @purvimisal (#10973)
• suggestion @jcqli (#11345)
• topic @vextor22 (#10967)
• user @jian-en (#11117)

Steps to refactor

• Extract all the validators related to a given model type for example activity into a new file named as <model-type>_validators.py for example activity_validators.py from prod_validators.py. Make sure you name the validators file exactly as <folder-name-in-above-list>_validators.py
• Extract all the test related to the model type from prod_validators_test.py to <model-type>_validators_test.py.
• Clean up the unused imports from the new validators file and existing prod validators file.
• Clean up the unused variables from the new validators file and existing prod validators file.
• Fix the file overview to have the correct explanation and copyright year.

Example PR: https://github.com/oppia/oppia/pull/10416 #10360

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[DubeySandeep]

Hi @archi15 & @jcqli , I’ve assigned you to the feedback & suggestion part of this issue. You can start working on it. Do let us know if you have any questions. Thanks! 😃

[vojtechjelinek]

@Koushikk-24 Could you please first complete your work on subtopic?