Add headings to the security settings page (#6661)

* Changes the headings' rank of the security settings section

This commit changes the existing headings' rank of the security settings section from level 6 to level 4.

* Renames the auth.change_password string into auth.security

The "Security" preferences' section used to be called "Change password". When it was renamed, the string name wasn't changed.
This commits changes auth.change_password to auth.security.

* Adds a heading to the password change form

There was previously no heading for the part of the "Security" page that contain the password change form.
This commit adds a rank 4 heading to this section and reintroduces an "auth.change_password" string to be used inside it.

* Removes useless HR elements

The various sections of the "Security" settings page were previously separated by HR elements.
Now that there is proper headings, they're not required anymore.

* Updates CSS

This commit updates CSS in such a way that the same style is applied to all the H4 elements of the settings.

* Correct a mistake

A character went missing on one of the previous commits, broking the CSS.
This new commit fixes it.

app/views/auth/registrations/_sessions.html.haml

@@ -1,4 +1,4 @@
-%h6= t 'sessions.title'
+%h4= t 'sessions.title'
 %p.muted-hint= t 'sessions.explanation'
 
 .table-wrapper

app/views/auth/registrations/edit.html.haml

@@ -1,6 +1,7 @@
 - content_for :page_title do
-  = t('auth.change_password')
+  = t('auth.security')
 
+%h4= t('auth.change_password')
 = simple_form_for(resource, as: resource_name, url: registration_path(resource_name), html: { method: :put, class: 'auth_edit' }) do |f|
   = render 'shared/error_messages', object: resource
 
@@ -15,12 +16,9 @@
   - else
     %p.hint= t('users.seamless_external_login')
 
-%hr/
-
 = render 'sessions'
 
 - if open_deletion?
-  %hr/
 
-  %h6= t('auth.delete_account')
+  %h4= t('auth.delete_account')
   %p.muted-hint= t('auth.delete_account_html', path: settings_delete_path)