attribute invite codes to admins/moderators

What does this MR do?

when a regular user (who has the appropriate permissions) creates an invite, we record that user's id in the createdById column (see https://activitypub.software/TransFem-org/Sharkey/-/blob/feature%2Fattribute-invites-to-mods/packages/backend/src/server/api/endpoints/invite/create.ts#L77-78 )

but when an admin/mod creates an invite via the control panel, we didn't

now we do

thanks to Rire on Discord for making me look

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added enhancement label

requested review from @CenTdemeern1, @julia, @Marie, and @tess

approved this merge request

approved this merge request

mentioned in commit 42d82796

merged