WindowsAuthProvider setting to restrict BUILTIN\Administrators

Some environments do not want to automatically allow Windows admin
accounts to be Gitblit admins.  This patch allows disabling/enabling the
relationship between Windows builtin admin accounts and Gitblit accounts.

releases.moxie

 	- Support Markdown image links relative to the repository root (issue-324)
 	- Added filesystem write permission check (issue-345)
 	- Added GO launch parameter for redirecting logging to a rolling, daily log file (issue-348)
+	- Added settings to Windows authentication provider to permit/prohibit BUILTIN\Administrators from being Gitblit Admins (issue-354)
 	- Support rendering confluence, mediawiki, textile, tracwiki, and twiki markup documents
 	- Added setting to globally disable anonymous pushes in the receive pack
 	- Added a normalized diffstat display to the commit, commitdiff, and compare pages
 	- { name: 'realm.ldap.synchronize', defaultValue: 'false' }
 	- { name: 'realm.ldap.syncPeriod', defaultValue: '5 MINUTES' }
 	- { name: 'realm.ldap.removeDeletedUsers', defaultValue: 'true' }
+	- { name: 'realm.windows.permitBuiltInAdministrators', defaultValue: 'true' }
 	- { name: 'web.commitMessageRenderer', defaultValue: 'plain' }
 	- { name: 'web.documents', defaultValue: 'readme home index changelog contributing submitting_patches copying license notice authors' }
 	- { name: 'web.showBranchGraph', defaultValue: 'true' }

src/main/distrib/data/gitblit.properties

 # SINCE 1.3.0
 realm.windows.allowGuests = false
 
+# Allow user accounts belonging to the BUILTIN\Administrators group to be
+# Gitblit administrators.
+#
+# SINCE 1.4.0
+realm.windows.permitBuiltInAdministrators = true
+
 # The default domain for authentication.
 #
 # If specified, this domain will be used for authentication UNLESS the supplied

src/main/java/com/gitblit/auth/WindowsAuthProvider.java

        		groupNames.add(group.getFqn());
         }
 
-        if (groupNames.contains("BUILTIN\\Administrators")) {
-        	// local administrator
-        	user.canAdmin = true;
+       	if (settings.getBoolean(Keys.realm.windows.permitBuiltInAdministrators, true)) {
+       		if (groupNames.contains("BUILTIN\\Administrators")) {
+       			// local administrator
+       			user.canAdmin = true;
+       		}
         }
 
         // TODO consider mapping Windows groups to teams