[FIX] project: reintroduce subtask_project_id field

The subtask_project_id field was removed by task #38498, amongst many
other changes.

This change makes some workflows much more difficult: for example, using
a different project with its own columns is kind of awkward.  It
requires many more clicks and custom filters to achieve the same result
as previously.

With this commit, we simply reintroduce the field and use it to set the
default project_id of a subtask.

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