[IMP] project: add 3 indexes to project_task

This commit adds 3 indexes targeting project_task which are not only
useful in regular RW use cases, but help tremendously when deleting
tasks.

In a production scenario, deleting a single task record went from 685ms
to 5ms.

Equivalent queries for an existing database:

CREATE INDEX account_analytic_line_task_id_index ON account_analytic_line(task_id);
CREATE INDEX project_task_parent_id_index ON project_task(parent_id);
CREATE INDEX sale_order_line_task_id_index ON sale_order_line(task_id);

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