FEATURE: improved tag and category watching and tracking

- present tags watched on the user prefs page
- automatically watch or unwatch old topics based on watch status

New watching and tracking logic takes care of handling old topics
(either with or without read state)

When you watch a topic you now watch historically

Also removes confusing warnings from user.

app/controllers/users_controller.rb

@@ -89,10 +89,6 @@ class UsersController < ApplicationController
     user = fetch_user_from_params
     guardian.ensure_can_edit!(user)
 
-    if params[:unwatch_category_topics]
-      TopicUser.unwatch_categories!(user, params[:unwatch_category_topics])
-    end
-
     if params[:user_fields].present?
       params[:custom_fields] = {} unless params[:custom_fields].present?