Combine 3 sign-in settings

What does this MR do and why?

We are combining these three settings, in this order:

1. Appearance.description: Admin > Settings > Appearance > Sign in/Sign up pages > Description
2. ApplicationSetting.sign_in_text: Admin > Settings > General > Sign-in restriction > Sign-in text
3. ApplicationSetting.help_text: Admin > Settings > Preferences > Sign-in and Help page > Additional text to show on the sign-in page

The surviving field will be Appearance.description

The API options were marked as deprecated in 16.2, !125271 (merged).

In this MR we remove the Appearance.description, ApplicationSetting.sign_in_text in the following fashion:

• Commit #1: database migration
• Commit #2: frontend field removal
• Commit #3: backend field removal (that are used in the API too)
• Commit #4: changes in the documentation

Changelog: changed EE: true

Closes Redesigned Login Page: Options to modify sign i... (#385648 - closed)

Closes Combine sign-in appearances settings (#410885 - closed)

Screenshots or screen recordings

There should be no changes in the other or content of the custom fields.

Changes in the documentation:

Before	After

How to set up and validate locally

1. Before checking this branch
   • Setup the three setting options above.
   • On an incognito window, take a look at the sign-in page: https://gdk.test:3443/users/sign_in
2. Checking the branch and apply migration script here.
3. The sign-in page should look identical.
4. The two last settings options should not be visible.

Changes in the documentation:

1. Check https://gdk.test:3443/help/user/admin_area/settings/sign_in_restrictions.md
2. Check https://gdk.test:3443/help/user/admin_area/settings/help_page.md
3. Check https://gdk.test:3443/help/api/settings.html

MR acceptance checklist

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