FEATURE: Improve composer warnings for mentions (#18796)

* FEATURE: Show warning if group cannot be mentioned

A similar warning is displayed when the user cannot be mentioned because
they have not been invited to the topic.

* FEATURE: Resolve mentions for new topic

This commit improves several improvements and refactors
/u/is_local_username route to a better /composer/mentions route that
can handle new topics too.

* FEATURE: Show warning if only some are notified

Sometimes users are still notified even if the group that was mentioned
was not invited to the message. This happens because its members were
invited directly or are members of other groups that were invited.

* DEV: Refactor _warnCannotSeeMention

app/controllers/users_controller.rb

@@ -497,93 +497,6 @@ class UsersController < ApplicationController
     end
   end
 
-  def is_local_username
-    usernames = params[:usernames] if params[:usernames].present?
-    usernames = [params[:username]] if params[:username].present?
-
-    raise Discourse::InvalidParameters.new(:usernames) if !usernames.kind_of?(Array) || usernames.size > 20
-
-    groups = Group.where(name: usernames).pluck(:name)
-    mentionable_groups =
-      if current_user
-        Group.mentionable(current_user, include_public: false)
-          .where(name: usernames)
-          .pluck(:name, :user_count)
-          .map do |name, user_count|
-          {
-            name: name,
-            user_count: user_count
-          }
-        end
-      end
-
-    usernames -= groups
-    usernames.each(&:downcase!)
-
-    users = User
-      .where(staged: false, username_lower: usernames)
-      .index_by(&:username_lower)
-
-    cannot_see = {}
-    here_count = nil
-
-    topic_id = params[:topic_id]
-    if topic_id.present? && topic = Topic.find_by(id: topic_id)
-      topic_muted_by = TopicUser
-        .where(topic: topic)
-        .where(user_id: users.values.map(&:id))
-        .where(notification_level: TopicUser.notification_levels[:muted])
-        .pluck(:user_id)
-        .to_set
-
-      if topic.private_message?
-        topic_allowed_user_ids = TopicAllowedUser
-          .where(topic: topic)
-          .where(user_id: users.values.map(&:id))
-          .pluck(:user_id)
-          .to_set
-
-        topic_allowed_group_ids = TopicAllowedGroup
-          .where(topic: topic)
-          .pluck(:group_id)
-          .to_set
-      end
-
-      usernames.each do |username|
-        user = users[username]
-        next if user.blank?
-
-        cannot_see_reason = nil
-        if !user.guardian.can_see?(topic)
-          cannot_see_reason = topic.private_message? ? :private : :category
-        elsif topic_muted_by.include?(user.id)
-          cannot_see_reason = :muted_topic
-        elsif topic.private_message? && !topic_allowed_user_ids.include?(user.id) && !user.group_ids.any? { |group_id| topic_allowed_group_ids.include?(group_id) }
-          cannot_see_reason = :not_allowed
-        end
-
-        if !guardian.is_staff? && cannot_see_reason.present? && cannot_see_reason != :private && cannot_see_reason != :category
-          cannot_see_reason = nil # do not leak private information
-        end
-
-        cannot_see[username] = cannot_see_reason if cannot_see_reason.present?
-      end
-
-      if usernames.include?(SiteSetting.here_mention) && guardian.can_mention_here?
-        here_count = PostAlerter.new.expand_here_mention(topic.first_post, exclude_ids: [current_user.id]).size
-      end
-    end
-
-    render json: {
-      valid: users.keys,
-      valid_groups: groups,
-      mentionable_groups: mentionable_groups,
-      cannot_see: cannot_see,
-      here_count: here_count,
-      max_users_notified_per_group_mention: SiteSetting.max_users_notified_per_group_mention
-    }
-  end
-
   def render_available_true
     render(json: { available: true })
   end