As part of this commit, a bug where updating a tag's notification level on the server side does not update the state of the user's tag notification levels on the client side is fixed too.
This is a recent regression introduced by https://github.com/discourse/discourse/pull/12937 which makes it so that when looking at a user profile that is not your own, specifically the category and tag notification settings, you would see your own settings instead of the target user. This is only a problem for admins because regular users cannot see these details for other users.
The issue was that we were using `scope` in the serializer, which refers to the current user, rather than using a scope for the target user via `Guardian.new(user)`.
However, on further inspection the `notification_levels_for` method for `TagUser` and `CategoryUser` did not actually need to be accepting an instance of Guardian, all that it was using it for was to check guardian.anonymous? which is just a fancy way of saying user.blank?. Changed this method to just accept a user instead and send the user in from the serializer.
The user may have changed their category or tag tracking settings since a topic was tracked/watched based on those settings in the past. In that case we need to alter the reason message we show them otherwise it is very confusing for the end user to be told they are tracking a topic because of a category, when they are no longer tracking that category.
For example: "You will see a count of new replies because you are tracking this category." becomes: "You will see a count of new replies because you were tracking this category in the past."
To do this, it was necessary to add tag and category tracking info to current user serializer. I improved the serializer code so it only does 3 SQL queries instead of 9 to get the tracking information for tags and categories for the current user.
A small fix for Basic User Serializers where some downstream serializers do not correctly set user objects. This caused some issues in certain plugins that depend on the user method to return a user.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
