[IMP] mail: update subscription instead of unlink / create new followers

In some cases when subscribing followers code may ask to force subtypes
of the subscription. It means existing followers should have only asked
subtypes independently from existing subtypes. Previously it was done
by unlinking and creating new followers for same partner or channel.
This commit proposes to instead update missing subtypes and remove extra
subtypes.

It has a small impact in real life as forcing subtype is not done often.
Some tests are improved thanks to this commit. This commit is linked to
task ID 1839043. Closes #24417.

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