Improve fetching of the donation amount, remove unecessary globals.
- Add User-Agent header when accessing wikimediafoundation.org to prevent 403s
- If remote information is for some reason unavailable, don't display the banner at all
- Remove globals that are no longer used
- Use $wgDBName as the default $wgCentralDBname for small wikis. Schema update will create tables in $wgDBName anyway.