Merge branch '23279-tabbing-over-the-login-screen-should-go-from-password-to-submit' into 'master'

Improve tabbing usability for sign in page

## What does this MR do?
Rearranged the html elements so that the `sign in` button gets tabbed first before the other elements and used CSS to rearrange the visual layout.

I did not use `tabindex` because it does not play well with screen readers and is not recommended from an accessibility standpoint.

## Are there points in the code the reviewer needs to double check?

*  Need to make sure that there aren't any side effects to my code change. `.new_user` seemed to be automatically added through `form_for()` and I'm not sure if there will be other cases where `.new_user` is used where there is no sign in page with forget your password.

## Why was this MR needed?
Improves the tabbing order

## Screenshots (if relevant)
None

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
Closes #23279

See merge request !6867

CHANGELOG

@@ -4,6 +4,7 @@ v 8.13.0 (unreleased)
   - Improve Merge When Build Succeeds triggers and execute on pipeline success. (!6675)
   - Respond with 404 Not Found for non-existent tags (Linus Thiel)
   - Truncate long labels with ellipsis in labels page
+  - Improve tabbing usability for sign in page (ClemMakesApps)
   - Adding members no longer silently fails when there is extra whitespace
   - Update runner version only when updating contacted_at
   - Add link from system note to compare with previous version

app/assets/stylesheets/pages/login.scss

@@ -48,6 +48,16 @@
       margin: 0 0 10px;
     }
 
+    .new_user {
+      position: relative;
+      padding-bottom: 35px;
+    }
+
+    .sign-in {
+      position: absolute;
+      bottom: 0;
+    }
+
     .login-footer {
       margin-top: 10px;
 

app/views/devise/sessions/_new_base.html.haml

 = form_for(resource, as: resource_name, url: session_path(resource_name)) do |f|
   = f.text_field :login, class: "form-control top", placeholder: "Username or Email", autofocus: "autofocus", autocapitalize: "off", autocorrect: "off"
   = f.password_field :password, class: "form-control bottom", placeholder: "Password"
+  .sign-in
+    = f.submit "Sign in", class: "btn btn-save"
   - if devise_mapping.rememberable?
     .remember-me.checkbox
       %label{for: "user_remember_me"}
@@ -8,5 +10,3 @@
         %span Remember me
       .pull-right
         = link_to "Forgot your password?", new_password_path(resource_name)
-  %div
-    = f.submit "Sign in", class: "btn btn-save"