Removes "NOT VALID" mark from the notifications -> template_history
foreign key constraint by validating existing records.
From postgres docs:
> This form validates a foreign key or check constraint that was
> previously created as NOT VALID, by scanning the table to ensure there
> are no rows for which the constraint is not satisfied. Nothing happens
> if the constraint is already marked valid.
> Validation can be a long process on larger tables. The value of
> separating validation from initial creation is that you can defer
> validation to less busy times, or can be used to give additional time
> to correct pre-existing errors while preventing new errors. Note also
> that validation on its own does not prevent normal write commands
> against the table while it runs.
> Validation acquires only a SHARE UPDATE EXCLUSIVE lock on the table
> being altered. If the constraint is a foreign key then a ROW SHARE
> lock is also required on the table referenced by the constraint.
