celery: allow forced synchronous execution

[valentijnscholten]

Currently our dojo_async_task decorator looks at the block_execution field in the users profile to determine if a task (deduplication, push to jira, send email, etc) should be performed synchronously, or asynchronously using celery.

There can be situation where this is the right default approach, for example when using the API. But sometimes in the UI you want direct feedback to see if that push to JIRA (or other task) was succesful.

I am going to use this in a future PR, but didn't want to sneak it in there. So here is the sync=True flag in its own PR.

example: jira_helper.push_to_jira(finding, sync=True) will ensure it's always performed synchronously in the current request/thread and the task won't get posted to celery.