Backport issuable for assignee slash commands from EE

Avoid conflicts when merge CE to EE by backporting code from EE.

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/issuable.rb

@@ -102,6 +102,18 @@ 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

@@ -92,9 +92,12 @@ module QuickActions
     desc 'Assign'
     explanation do |users|
-      "Assigns #{users.first.to_reference}." if users.any?
+      users = issuable.allows_multiple_assignees? ? users : users.take(1)
+      "Assigns #{users.map(&:to_reference).to_sentence}."
+    end
+    params do
+      issuable.allows_multiple_assignees? ? '@user1 @user2' : '@user'
     end
-    params '@user'
     condition do
       current_user.can?(:"admin_#{issuable.to_ability_name}", project)
     end
@@ -104,25 +107,44 @@ module QuickActions
     command :assign do |users|
       next if users.empty?
-      if issuable.is_a?(Issue)
+      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 'Remove assignee'
+    desc do
+      if issuable.allows_multiple_assignees?
+        'Remove all or specific assignee(s)'
+      else
+        'Remove assignee'
+      end
+    end
     explanation do
-      "Removes assignee #{issuable.assignees.first.to_reference}."
+      "Removes #{'assignee'.pluralize(issuable.assignees.size)} #{issuable.assignees.map(&:to_reference).to_sentence}."
+    end
+    params do
+      issuable.allows_multiple_assignees? ? '@user1 @user2' : ''
     end
     condition do
       issuable.persisted? &&
         issuable.assignees.any? &&
         current_user.can?(:"admin_#{issuable.to_ability_name}", project)
     end
-    command :unassign do
-      if issuable.is_a?(Issue)
-        @updates[:assignee_ids] = []
+    command :unassign do |unassign_param = nil|
+      # 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.supports_multiple_assignees?
+        @updates[:assignee_ids] =
+          if users&.any?
+            issuable.assignees.pluck(:id) - users.map(&:id)
+          else
+            []
+          end
       else
         @updates[:assignee_id] = nil
       end