celery_backend/config: Enable to set Sentry DSN per task type

Add a task_prerun celery signal handler in order to set Sentry DSN based
on task name. The mapping between a task name and its DSN must be stored
in configuration under a "task_sentry_dsn" key.

For this feature to work, no SWH_SENTRY_DSN environment variable should
be defined as it overrides the sentry_dsn value passed to init_sentry
function.

Related to swh/meta#4949.