DEV: Merge category and tag hashtags code paths (#10216)

Category and tag hashtags used to be handled differently even though
most of the code was very similar. This design was the root cause of
multiple issues related to hashtags.

This commit reduces the number of requests (just one and debounced
better), removes the use of CSS classes which marked resolved hashtags,
simplifies a lot of the code as there is a single source of truth and
previous race condition fixes are now useless.

It also includes a very minor security fix which let unauthorized users
to guess hidden tags.

lib/pretty_text/helpers.rb

@@ -4,6 +4,8 @@ module PrettyText
   module Helpers
     extend self
 
+    TAG_HASHTAG_POSTFIX = "::tag"
+
     # functions here are available to v8
     def t(key, opts)
       key = "js." + key
@@ -102,13 +104,12 @@ module PrettyText
     end
 
     def category_tag_hashtag_lookup(text)
-      tag_postfix = '::tag'
-      is_tag = text =~ /#{tag_postfix}$/
+      is_tag = text =~ /#{TAG_HASHTAG_POSTFIX}$/
 
       if !is_tag && category = Category.query_from_hashtag_slug(text)
         [category.url, text]
       elsif (!is_tag && tag = Tag.find_by(name: text)) ||
-            (is_tag && tag = Tag.find_by(name: text.gsub!("#{tag_postfix}", '')))
+            (is_tag && tag = Tag.find_by(name: text.gsub!(TAG_HASHTAG_POSTFIX, '')))
         ["#{Discourse.base_url}/tag/#{tag.name}", text]
       else
         nil