Slash command interpret service asks issuable for assignee capabilities

Instead of checking in `SlashCommands::InterpretService` what the issuable the
type of the issuable is, ask the issuable if it is capable to do those thing and
implement it in the issuable itself. The issuable will check if it's possible
and if the licensed feature is available.

This should also make it easier to ever add multiple assignees to MergeRequests.

app/models/concerns/ee/issuable.rb

+module EE
+  module Issuable
+    extend ActiveSupport::Concern
+
+    def allows_multiple_assignees?
+      supports_multiple_assignees? &&
+        is_a?(Issue) && project.feature_available?(:multiple_issue_assignees)
+    end
+  end
+end

app/models/concerns/issuable.rb

@@ -105,6 +105,18 @@ module Issuable
     def locking_enabled?
       title_changed? || description_changed?
     end
+
+    def allows_multiple_assignees?
+      false
+    end
+
+    def has_multiple_assignees?
+      supports_multiple_assignees? && assignees.count > 1
+    end
+
+    def supports_multiple_assignees?
+      respond_to?(:assignee_ids)
+    end
   end
 
   module ClassMethods

app/services/quick_actions/interpret_service.rb

@@ -92,13 +92,11 @@ module QuickActions
 
     desc 'Assign'
     explanation do |users|
-      ## EE-specific
-      users = issuable.is_a?(Issue) ? users : users.take(1)
+      users = issuable.allows_multiple_assignees? ? users : users.take(1)
       "Assigns #{users.map(&:to_reference).to_sentence}."
     end
     params do
-      ## EE-specific
-      issuable.is_a?(Issue) ? '@user1 @user2' : '@user'
+      issuable.allows_multiple_assignees? ? '@user1 @user2' : '@user'
     end
     condition do
       current_user.can?(:"admin_#{issuable.to_ability_name}", project)
@@ -109,26 +107,27 @@ module QuickActions
     command :assign do |users|
       next if users.empty?
 
-      if issuable.is_a?(Issue)
-        # EE specific. In CE we should replace one assignee with another
+      if issuable.allows_multiple_assignees?
         @updates[:assignee_ids] = issuable.assignees.pluck(:id) + users.map(&:id)
+      elsif issuable.supports_multiple_assignees?
+        @updates[:assignee_ids] = [users.last.id]
       else
         @updates[:assignee_id] = users.last.id
       end
     end
 
     desc do
-      if issuable.is_a?(Issue)
+      if issuable.allows_multiple_assignees?
         'Remove all or specific assignee(s)'
       else
         'Remove assignee'
       end
     end
     explanation do
-      "Removes #{'assignee'.pluralize(issuable.assignees.size)} #{issuable.assignees.map(&:to_reference).to_sentence}"
+      "Removes #{'assignee'.pluralize(issuable.assignees.size)} #{issuable.assignees.map(&:to_reference).to_sentence}."
     end
     params do
-      issuable.is_a?(Issue) ? '@user1 @user2' : ''
+      issuable.allows_multiple_assignees? ? '@user1 @user2' : ''
     end
     condition do
       issuable.persisted? &&
@@ -136,11 +135,12 @@ module QuickActions
         current_user.can?(:"admin_#{issuable.to_ability_name}", project)
     end
     command :unassign do |unassign_param = nil|
-      users = extract_users(unassign_param)
+      # When multiple users are assigned, all will be unassigned if multiple assignees are no longer allowed
+      users = extract_users(unassign_param) if issuable.allows_multiple_assignees?
 
-      if issuable.is_a?(Issue)
+      if issuable.supports_multiple_assignees?
         @updates[:assignee_ids] =
-          if users.any?
+          if users&.any?
             issuable.assignees.pluck(:id) - users.map(&:id)
           else
             []
@@ -156,7 +156,7 @@ module QuickActions
     end
     params '@user1 @user2'
     condition do
-      issuable.is_a?(Issue) &&
+      issuable.allows_multiple_assignees? &&
         issuable.persisted? &&
         issuable.assignees.any? &&
         current_user.can?(:"admin_#{issuable.to_ability_name}", project)