Add server-side validation to Subscription APIs

Description

The server needs to perform its own validation to ensure that only valid user data is accepted and stored in the KV store.

QA Test Steps

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Reporter

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Mana

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Assignee

Mattermost Community

QA Assignee

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Reporter

Michael Kochell

Epic Link

Fix versions

Mattermost Team

Integrations

Sprint

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Labels

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QA Testing Areas

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Components

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Configure