Bypass signup domain validation for external users

What does this MR do?

Bypass signup domain validation when adding an external user

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

From #25279 (closed)

When a company has a hosted GitLab, it makes sense to Whitelist the company domain to ensure that only users (in this case coming through google oauth) from the company are able to gain access. However, this also blocks an admin when creating a new external user.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

• #25279 (closed)

marked the task All builds are passing as completed

unmarked as a Work In Progress

@remohammadi Thank you for the contribution, could you please also add a corresponding test in spec/models/user_spec.rb? Please search for when no signup domains whitelisted for relevant tests.

added ~164274 ~15078 ~17488 labels

assigned to @remohammadi

marked as a Work In Progress

mentioned in issue #27028 (moved)

added 575 commits

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• eb885f0d...1fe80c29 - 7 commits from branch gitlab-org:master
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marked the task Added for this feature/bug as completed

I've added some tests for these changes. @godfat

unmarked as a Work In Progress

added ~480950 and removed ~15078 labels

resolved all discussions

assigned to @godfat

@remohammadi Sorry for the delay. I was ill and then hitting into holidays. Looks great to me, thank you! I left a minor comment about the phrasing.

@rspeicher Could you please take the final review? Thanks!

assigned to @rspeicher

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resolved all discussions

@remohammadi Just a couple spec quibbles. Thanks!

assigned to @remohammadi

added 543 commits

• 339cac4b...a965edb8 - 542 commits from branch gitlab-org:master
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@rspeicher Fixed all of them except "not-allowed".

@remohammadi Responded to your point, thanks!

added 132 commits

• 48bb5299...bbb7fbcd - 131 commits from branch gitlab-org:master
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resolved all discussions

assigned to @rspeicher

changed milestone to %9.0

merged

mentioned in commit b28d66c3

@remohammadi Thanks!

mentioned in issue #3202 (closed)

mentioned in issue gitlab#7899