use "app_api" session key in rate limit middleware, "app_api_system" is deprecated

[andrey18106]

This PR updating RateLimit middleware.

The app_api_system session key is deprecated and will be removed in the future. Use app_api session key instead.

• Resolves: #

Summary

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[bigcat88]

I corrected the logic of this so that the RateLimit would be applied to ExApps if the userId is in the session, so that the development of ExApps would be compatible with the rate limit.

For ExApps, it is impossible to check on their side what the rate limit is, and in this case, if the request comes from a user, ExApp must set the userId and this will be checked by the server as usual.

If they need it outside the context of the user, they may not expose the user if the endpoint supports it and then the rate limit will be skipped.

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