This field adds the possibility to set the update date when modifying
an issue through the API.
A 'NoAutoDate' in-memory field is added in the Issue struct.
If the update_at field is set, NoAutoDate is set to true and the
Issue's UpdatedUnix field is filled.
That information is passed down to the functions that actually updates
the database, which have been modified to not auto update dates if
requested.
A guard is added to the 'EditIssue' API call, to checks that the
udpate_at date is between the issue's creation date and the current
date (to avoid 'malicious' changes). It also limits the new feature
to project's owners and admins.
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