[ADD] project_timesheet, sale_service: support for project sub-tasks

This commit introduces "Sub-task", a concept remotely similar to
the "delegated tasks" available in version 7.0, but covering a
more specific and narrow scope.

Activating Sub-tasks
--------------------

Sub-tasks are made available within any project by configuring
a "sub-task project" in the project settings. This feature is
considered low-level and technical, and only accessible with
"Technical Features" enabled.
The "sub-task project" may be the project itself.

Using Sub-tasks
---------------

Conceptually, a sub-task represents the allocation of a part
of the scope of its parent task. Multiple sub-task levels
are possible.

Once activated in project settings, a new "Sub-tasks" smart button
will appear on the task form, giving the option to view and
create sub-tasks. In addition, a new field "Sub-tasks Hours" will
appear next to the other computed "Hours" fields, representing
the total hours allocated to sub-tasks.
(Technically, the sum of the maximum between the planned hours
and spent hours of all sub-tasks, with spent hours always used
when a task is closed)
These sub-task hours will be dynamically subtracted from the
remaining hours of the task, and will therefore affect the
task's progress as well.

Upon creation, a sub-task will inherit the link to the
original sales order and procurement from its parent task, and
will by default be assigned to the "sub-task project" set on
the project of the parent task.

Enhancement
-----------
The kanban view for tasks will now display a color-coded label
with the number of remaining hours, whenever the percentage
of remaining hours is below 20% (orange=20%-, red=1%-).

Note: This is a forward port of the commit 9eac32beeabacf01feaba1f4712bfbea586a9035

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