BE: Add column for 'bypass placeholder confirmation'

What does this MR do and why?

The parent issue is for adding two Import-related settings at the group-level restricted to groups containing enterprise users:

• turning on the ability to skip confirmation emails that go to users during the placeholder user assignment process
• an expiry date for the above setting, as suggested here

All the issues related to this feature are behind the flag importer_user_mapping_allow_bypass_of_confirmation

This current MR

• saves the allow_enterprise_bypass_placeholder_confirmation column as well as saving value in response to the request from the frontend

Subsequent MRs/Issues address

• implementing the logic for the expiry date feature #540573 (closed)
• license or enterprise user check and implementing the bypass of user confirmation as a result of allow_bypass_placeholder_confirmation_for_enterprise being true, in accordance with expiration date - #542284 (closed)
• the frontend for the feature

SQL

ALTER TABLE "namespace_settings"
    ADD COLUMN "allow_enterprise_bypass_placeholder_confirmation" boolean NOT NULL DEFAULT FALSE;

References

Parent issue #534329 (closed)

Epic &17382 (closed)

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Before	After

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Related to #534329 (closed)