[migractions] add a dropped_constraints() migration command
Example usage:
>>> with dropped_constraints('MyType', 'myattr',
... UniqueConstraint, droprequired=True):
... add_attribute('MyType', 'myattr')
... # + instructions to fill MyType.myattr column
...
>>>
cubicweb-web: embedded-javascript-library usr/share/cubicweb/cubes/shared/data/jquery.js