Mattermost-server: [Help Wanted] [MM-9956] Remove last uses of IntegrationStore and delete the store

[esethna]

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Notes: Jira ticket

Over the last year we've been slowly moving away from using our Flux stores and towards using redux. More info on that here.

Now we're really close to being able to remove the IntegrationStore, which is one of those old Flux stores. The last component using it is ConfirmIntegration.

For this ticket:

1. Replace the usage of IntegrationStore by passing in the redux state as a prop in the component's index.js file
2. Delete the stores/integration_store.jsx file
3. (Bonus) Also remove usage of UserStore in the ConfirmIntegration component, using redux instead

This component migration guide should have some helpful info if you need it.
https://docs.mattermost.com/developer/webapp-to-redux.html#component-migration-steps

Let Joram (@joram on pre-release, @jwilander on GitHub) know if you have any questions

[n1aba]

I would like to start working on this