Add custom brand text on new project pages

What does this MR do?

Adds a new Markdown text field in the appearance administration which is displayed on new project pages.

This development is sponsored by Siemens (/cc @bufferoverflow)

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

• In the Appearance model title and description are now optional, not sure exactly why they were required before. Now it's possible to just change the logo or new project description, without having to specify a title and description for the login page as well.
  • Let me know if I should change the DB fields to be nullable as well
• I moved the "Navigation bar" section to the top, to keep the bigger form fields at the bottom
• The preview in the administration only displays the login page, let me know if I should add a preview for the new project page as well (probably as a separate button)
• Not sure how much docs/tests/i18n is needed for this, let me know if something's missing :)

Why was this MR needed?

Provide a space to indicate instance-specific project guidelines.

Screenshots (if relevant)

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Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/15265