XWIKI-10212: Implement a (Color) Theme Application for Flamingo.
XWIKI-10772: Subwikis does not have any setting by default

* Restoring XWiki.XWikiPreferences in xwiki-enterprise-ui-wiki
* Updating XWiki.XWikiPreferences in xwiki-enterprise-ui-admin (which should be in xwiki-enterprise-ui-mainwiki IMO)
* Setting FontAwesome as the default icon theme
* Setting the new Flamingo theme as the default color theme