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:

1. create ticket T1
2. create T2
3. open the page for T1, click "edit blocking tags"
4. 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.

Actual results:

• 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.

Original report: https://phabricator.wikimedia.org/T76556