Add note error handler as entries for JH overriding

What does this MR do and why?

In JH we need to block illegal content when users are editing/creating/replying a note and give them a error flash with useful message, so a encapsulated error message method like https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/notes/components/comment_form.vue#L190-196 in vue component will be helpful for JH to override it.

Following the same way app/assets/javascripts/notes/components/comment_form.vue did, this MR is encapsulating the error handler to make the feature mentioned can work when users are editing/replying a note.

related JH MR: gitlab-jh/gitlab!194 (merged)

related JH issue: https://gitlab.com/gitlab-jh/gitlab/-/issues/358

Screenshots or screen recordings

How to set up and validate locally

1. Checkout the branch in gitlab-jh/gitlab!194 (merged)
2. paste this patch

diff --git a/app/controllers/concerns/notes_actions.rb b/app/controllers/concerns/notes_actions.rb
--- a/app/controllers/concerns/notes_actions.rb	(revision 3d6143e41260cac47289c35fafa651cab3efaf91)
+++ b/app/controllers/concerns/notes_actions.rb	(date 1638244961543)
@@ -81,6 +81,15 @@
       return
     end
 
+    if @note.invalid?
+      json = {
+        valid: false,
+        errors: note.errors.full_messages.join('. ')
+      }
+      render json: json, status: :forbidden
+      return
+    end
+
     prepare_notes_for_rendering([@note])
 
     respond_to do |format|

diff --git a/app/models/note.rb b/app/models/note.rb
--- a/app/models/note.rb	(revision 3d6143e41260cac47289c35fafa651cab3efaf91)
+++ b/app/models/note.rb	(date 1638180191454)
@@ -101,6 +101,9 @@
 
   validate :does_not_exceed_notes_limit?, on: :create, unless: [:system?, :importing?]
 
+  validates_each :note do |record, attr, value|
+    record.errors.add(attr, 'has invalid content')
+  end
   # @deprecated attachments are handled by the Upload model.
   #
   # https://gitlab.com/gitlab-org/gitlab/-/issues/20830

3. create/edit/reply the note anywhere

MR acceptance checklist

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