Replace prepend-bottom-20 in devise

This replaces the prepend-bottom-20 css
class in the devise folder, the files are
all related to the signup/signin process

app/views/devise/confirmations/almost_there.haml

-.well-confirmation.text-center.append-bottom-20
+.well-confirmation.text-center.gl-mb-6
   %h1.gl-mt-0
     Almost there...
-  %p.lead.append-bottom-20
+  %p.lead.gl-mb-6
     Please check your email to confirm your account
   %hr
 - if Gitlab::CurrentSettings.after_sign_up_text.present?
@@ -9,6 +9,6 @@
     = markdown_field(Gitlab::CurrentSettings, :after_sign_up_text)
 %p.text-center
   No confirmation email received? Please check your spam folder or
-.append-bottom-20.prepend-top-20.text-center
+.gl-mb-6.prepend-top-20.text-center
   %a.btn.btn-lg.btn-success{ href: new_user_confirmation_path }
     Request new confirmation email

app/views/devise/shared/_signup_box.html.haml

@@ -23,7 +23,7 @@
     .form-group
       = f.label :email, class: 'label-bold'
       = f.email_field :email, value: @invite_email, class: 'form-control middle', data: { qa_selector: 'new_user_email_field' }, required: true, title: _('Please provide a valid email address.')
-    .form-group.append-bottom-20#password-strength
+    .form-group.gl-mb-5#password-strength
       = f.label :password, class: 'label-bold'
       = f.password_field :password, class: 'form-control bottom', data: { qa_selector: 'new_user_password_field' }, required: true, pattern: ".{#{@minimum_password_length},}", title: s_('SignUp|Minimum length is %{minimum_password_length} characters.') % { minimum_password_length: @minimum_password_length }
       %p.gl-field-hint.text-secondary= s_('SignUp|Minimum length is %{minimum_password_length} characters.') % { minimum_password_length: @minimum_password_length }

app/views/devise/unlocks/new.html.haml

@@ -4,7 +4,7 @@
     = form_for(resource, as: resource_name, url: unlock_path(resource_name), html: { method: :post, class: 'gl-show-field-errors' }) do |f|
       .devise-errors
         = render "devise/shared/error_messages", resource: resource
-      .form-group.append-bottom-20
+      .form-group.gl-mb-6
         = f.label :email
         = f.email_field :email, class: 'form-control', autofocus: 'autofocus', autocapitalize: 'off', autocorrect: 'off', title: 'Please provide a valid email address.'
       .clearfix