DEV: Remove enable_experimental_hashtag_autocomplete logic (#22820)

This commit removes any logic in the app and in specs around
enable_experimental_hashtag_autocomplete and deletes some
old category hashtag code that is no longer necessary.

It also adds a `slug_ref` category instance method, which
will generate a reference like `parent:child` for a category,
with an optional depth, which hashtags use. Also refactors
PostRevisor which was using CategoryHashtagDataSource directly
which is a no-no.

Deletes the old hashtag markdown rule as well.

spec/integrity/common_mark_spec.rb

@@ -5,9 +5,6 @@ RSpec.describe "CommonMark" do
     SiteSetting.enable_markdown_typographer = false
     SiteSetting.highlighted_languages = "ruby|aa"
 
-    # TODO (martin) Remove when enable_experimental_hashtag_autocomplete is default for all sites
-    SiteSetting.enable_experimental_hashtag_autocomplete = false
-
     html, state, md = nil
     failed = 0
 
@@ -34,7 +31,7 @@ RSpec.describe "CommonMark" do
           cooked = PrettyText.markdown(md, sanitize: false)
           cooked.strip!
           cooked.gsub!(" class=\"lang-auto\"", "")
-          cooked.gsub!(%r{<span class="hashtag">(.*)</span>}, "\\1")
+          cooked.gsub!(%r{<span class="hashtag-raw">(.*)</span>}, "\\1")
           cooked.gsub!(%r{<a name="(.*)" class="anchor" href="#\1*"></a>}, "")
           # we support data-attributes which is not in the spec
           cooked.gsub!("<pre data-code-startline=\"3\">", "<pre>")