Separate label subscription emails from label change email

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Description

Right now when we subscribe to a label we receive an email like this:

Something_to_be_improved

which apparently is the same as to when we change the labels in an issue/MR.

A label subscription email should probably contain more information other than informing that a label was added.

Proposal

Separate a label subscription email from a label change email.

\cc @JobV @DouweM

Edited by 🤖 GitLab Bot 🤖