Default theme favicon for administration theme

Problem/Motivation

Steps to reproduce the problem:

1. Create few sites.
2. Use the same core theme for administration pages.
3. Use different contrib themes for each site as site default theme.
4. Open the "Configuration" administration page of each site in separate browser tabs.
5. Add new empty browser tabs while you should not see only the "Configuration" word in the title of sites tabs.
6. Open some empty tab.
7. When you just looking at the title of sites tabs you cannot understand what site you see in some tab because you see default Drupal favicon and the "Configuration" word in all sites tabs.

Proposed resolution

Add possibility to use favicon of default theme on administration theme pages when these themes are different.

Remaining tasks

Add test
Before and After screenshots

User interface changes

Add the "Favicon" checkbox in the "Administration theme" section of the "Appearance" administrative page.

API changes

None

Data model changes

None