Add custom type for `window` object

Description

We have a few fields that we add to the `window` object in JS for use in other parts of the code base such as those in entry.js, but those cause problems with TypeScript. We should look at either how to define our own type for `window` or how to get rid of those fields ourselves.

QA Test Steps

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Status

Mana

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Assignee

Unassigned

QA Assignee

None

Reporter

Harrison Healey

Epic Link

None

Fix versions

Mattermost Team

Apps

Sprint

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QA Testing Areas

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GitHub Issue

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