Move constraint creation to SQL

[mathemancer]

Related to #2690

This PR removes SQLAlchemy from constraint creation calls, and adds an add_constraints function on the database that can add constraints to a preexisting table.

Technical details

Removes SQLAlchemy from the files:

• db/constraints/base.py
• db/constraints/operations/create.py
• db/constraints/utils.py
• mathesar/api/serializers/constraints.py

This also adds a number of SQL layer tests to cover constraint adding and copying, and removes some python layer tests that were made redundant.

NOTE Not all python-layer functions have new docstrings, since we're going to rework the python layer when solving #2850 .

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  visible errors.

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[silentninja]

Looks good to me. The test cases look comprehensive but I found the test case names hard to parse on the first look.