Stalker

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* **New:** ``ProjectUser`` now also holds a new field called ``rate``. The
default value is equal to the ``ProjectUser.user.rate``. It is a way to hold
the rate of a user on a specific project.

* **New:** Added the ``Invoice`` class.

* **New:** Added the ``Payment`` class.

* **New:** Added two simple mixins ``AmountMixin`` and ``UnitMixin``.

* **Update:** ``Good`` class is now mixed in with the new ``UnitMixin`` class.

* **Update:** ``BudgetEntry`` class is now mixed in with the new
``AmountMixin`` and ``UnitMixin`` classes.

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* **New:** ``Group`` permissions can now be set on ``__init__()`` with the
``permissions`` argument.

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* **Fix:** As usual after a new release that changes database schema, fixed the
corresponding Alembic revision (92257ba439e1).

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* **New:** ``Budget`` instances are now statusable.

* **Update:** Updated documentation to include database migration instructions
with Alembic.

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* **Fix:** Fixed a typo in the error message in
``User._validate_email_format()`` method.

* **Fix:** Fixed a query-invoked auto-flush problem in
``Task.update_parent_statuses()`` method.

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* **Fix:** Fixed alembic revision (f2005d1fbadc), it will now drop any existing
constraints before re-creating them. And the downgrade function will not
remove the constraints.