Convert the LDAP sign-in form to Pajamas

What does this MR do and why?

Convert the LDAP sign-in form to Pajamas

• Fixed #397684 (closed)
• Added autocomplete username and current-password, as recommended here: https://www.chromium.org/developers/design-documents/form-styles-that-chromium-understands
• Eliminated various errors in the console about multiple elements with the same ID.
• Converted the form and the submit button to Pajamas. Simplify the checkbox.
• Internationalize and simplify some strings.
• Removed padding in each individual element by adding it to the parent form.
• Removed .submit-container class and moved the remember me button closer to the submit button.

This is part 2 of a series of MRs to clean-up, improve and normalize the structure of the sign-in pages.

Changelog: changed

Screenshots or screen recordings

Restyle enabled

Before	After

Restyle disabled

Before	After

How to set up and validate locally

1. Enable LDAP following this guide
2. Play with both, new and old styles: echo "Feature.disable(:restyle_login_page)" | gdk rails console
3. [Optional] Enable the Admin Mode in Admin > Settings > General > Sign-in restrictions
4. On an incognito window, go to the main page: https://gdk.test:3443/admin
5. [Optional] Enter the admin area: https://gdk.test:3443/admin

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.