Send a notification to watching users when adding tag

[simon]

@lw1

I tested it again by creating a post and then adding the staff tag to it. The watching user can see the post (and the staff tag) in the New and Latest list, but they don’t have any notification in their notification list and no E-Mail was sent.

I have just tested this with tags on my local development site and on https://try.discourse.org/. When a user is watching a tag, they will receive a notification for topics that are given the tag when the topic is first created, but editing an existing topic to add a watched tag is not creating a notification.

When a user is watching a category, recategorizing an existing topic to the watched category is creating a notification for me on https://try.discourse.org/.