Signed-off-by: Rémy Coutable <remy@rymai.me>

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Closes #14885

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Signed-off-by: Rémy Coutable <remy@rymai.me>

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Signed-off-by: Rémy Coutable <remy@rymai.me>

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Closes #14899

Resolves #14411.

See merge request !3464

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

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Signed-off-by: Rémy Coutable <remy@rymai.me>

Closes #14875

Closes #13957

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Signed-off-by: Rémy Coutable <remy@rymai.me>

I’ve removed everything related to the feature based on this commit:
ce08f919f34fd8849834365

Resolves #10857.

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Signed-off-by: Rémy Coutable <remy@rymai.me>

Also, streamline the view.
Signed-off-by: Rémy Coutable <remy@rymai.me>

Signed-off-by: Rémy Coutable <remy@rymai.me>

Closes #3250

Closes #14676

Closes #14580

Improve and finish the fallback to the In-Reply-To and References header for the reply-by-email feature

A few things to note:
- The IncomingEmail feature is now enabled even without a
  correctly-formatted sub-address
- Message-ID for new thread mail are kept the same so that subsequent
  notifications to this thread are grouped in the thread by the email
  service that receives the notification
  (i.e. In-Reply-To of the answer == Message-ID of the first thread message)
- To maximize our chance to be able to retrieve the reply key, we look
  for it in the In-Reply-To header and the References header
- The pattern for the fallback reply message id is "reply-[key]@[gitlab_host]"
- Improve docs thanks to Axil

Closes #14528

Fixes #14585.

Also the specs have a minor improvement. Mainly the access right spec.
Changes are reflected in the docs

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>