Add api rate limit

[SomewhatDamaged]

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• I understand my contributions may be rejected for any reason

• I understand my contributions are for the benefit of this imageboard software

• I understand my contributions are licensed under the GNU AGPLv3

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This adds a 30s rate limit to the CREATE /api/v1/json/image endpoint. Note: seeing as this uses the limit_plug, anyone with staff Role will be exempt from it.

If you want to change the timeout, please see line 11 in image_controller.ex

[basisbit]

Thank you for this PR! Would this limit to one image per 30 seconds?

[SomewhatDamaged]

One access of the endpoint every 30 seconds, yeah.