Add support for unique constraints to Meta.constraints

[Simon Charette]

Just like Meta.indexes[Index] allows more granularity over Field.db_index=True/ Meta.index_together we should make Meta.constraints[UniqueConstraint] an analog for Field.unique=True/Meta.unique_together.

This ticket proposes to introduce django.db.constraint.UniqueConstraint(fields, name) to allow such constraints to be defined with the sole extra feature of allowing a name to be specified.

The mid-term goal of this feature addition is to rely on the partial indices work (#29547) that is likely to land in 2.2 to allow partial unique constraints to be defined which is a really useful feature.

e.g.

class Tweet(models.Model):
    user = models.ForeignKey(User, models.CASCADE)
    pinned = models.BooleanField(default=False)

    class Meta:
        constraints = [
            UniqueConstraint(
                fields=['user', 'pinned'],
                condition=Q(pinned=True),
                name='pinned_tweet'
            ),
        ]

Adding support for partial constraints should be tracked in a future ticket as this one will focus on refactoring the CheckConstraint work added for #11964 to the definition of different type of constraints.

[Simon Charette]

Ian started adjusting my initial PR in ​https://github.com/django/django/pull/10337.

I'd be great to get the commits referencing #29641 in before the 2.2 release as they are mostly adjustments that will be hard to perform if we commit to the currently documented interface.

I'd review it but since I've written a large chunk of this code it's a bit hard for me to do it.

I'm deassigning the ticket given Ian's work so far, feel free to claim the ticket Ian.

[Tim Graham <timograham@…>]

In 24dc7d8:

Refs #29641 -- Extracted reusable CheckConstraint logic into a base class.

[Tim Graham <timograham@…>]

In dba4a63:

Refs #29641 -- Refactored database schema constraint creation.

Added a test for constraint names in the database.

Updated SQLite introspection to use sqlparse to allow reading the
constraint name for table check and unique constraints.

Co-authored-by: Ian Foote <python@…>

[Tim Graham <timograham@…>]

In db13bca6:

Fixed #29641 -- Added support for unique constraints in Meta.constraints.

This constraint is similar to Meta.unique_together but also allows
specifying a name.

Co-authored-by: Ian Foote <python@…>