FEATURE: Sort hashtags starting with term higher priority (#19463)

This introduces another "section" of queries to the
hashtag autocomplete search, which returns results for
each type that start with the search term. So now results
will be in this order, and within these sections ordered
by the types in priority order:

1. Exact matches sorted by type
2. "starts with" sorted by type
3. Everything else sorted by type then name within type

lib/discourse_tagging.rb

@@ -13,6 +13,10 @@ module DiscourseTagging
           ON tgm.tag_group_id = tg.id
   SQL
 
+  def self.term_types
+    @term_types ||= Enum.new(contains: 0, starts_with: 1)
+  end
+
   def self.tag_topic_by_names(topic, guardian, tag_names_arg, append: false)
     if guardian.can_tag?(topic)
       tag_names = DiscourseTagging.tags_for_saving(tag_names_arg, guardian) || []
@@ -262,6 +266,7 @@ module DiscourseTagging
 
   # Options:
   #   term: a search term to filter tags by name
+  #   term_type: whether to search by "starts_with" or "contains" with the term
   #   limit: max number of results
   #   category: a Category to which the object being tagged belongs
   #   for_input: result is for an input field, so only show permitted tags
@@ -355,9 +360,15 @@ module DiscourseTagging
     term = opts[:term]
     if term.present?
       term = term.gsub("_", "\\_").downcase
-      builder.where("LOWER(name) LIKE :term")
       builder_params[:cleaned_term] = term
-      builder_params[:term] = "%#{term}%"
+
+      if opts[:term_type] == DiscourseTagging.term_types[:starts_with]
+        builder_params[:term] = "#{term}%"
+      else
+        builder_params[:term] = "%#{term}%"
+      end
+
+      builder.where("LOWER(name) LIKE :term")
       sql.gsub!("/*and_name_like*/", "AND LOWER(t.name) LIKE :term")
     else
       sql.gsub!("/*and_name_like*/", "")