Fix inability of updating Todo if .is_done but no more doable.

[plomtask] / migrations / 5_create_process_step_suppressions.sql

CREATE TABLE process_step_suppressions (
    process INTEGER NOT NULL,
    process_step INTEGER NOT NULL,
    PRIMARY KEY (process, process_step),
    FOREIGN KEY (process) REFERENCES processes(id),
    FOREIGN KEY (process_step) REFERENCES process_steps(id)
);