Use base64-encoding for login background to avoid encoding issues.

[dennisoelkers]

Note: This needs a backport to 6.3.

Description

Motivation and Context

This PR is using base64-encoding for the custom login background to avoid encoding issues with URL-encoding.

/nocl Changing internal feature.

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.