As a user, I want to see when a task is being marked/unmarked as blocking another task, and receive the corresponding notification if I'm subscribed.
Currently only the blocked task logs the action and notifies.
Steps to reproduce:
# create ticket `T1`
# create `T2`
# open the page for `T1`, click "edit blocking tags"
# add `T2` as blocker
Expected results: there will be a record on the `T1` page telling who and when added the blocker, and a similar record on the `T2` page for the dependency.
* `T1` logs "(someone) added a blocking task: `T2`:...". A notification is created
* However, `T2` doesn't log anything and its subscribers will miss the event.
The same happens when removing blocking tasks.