Move support for 'google-site-verification' to LocalSettings.php

[kghbln]

As of version 1.1.1 it is possible to add the google-site-verification tag to any page though this is basically only useful on main page. However this is bad for the following reasons:

• Everybody able to edit main page may do so
• It allows for inserting arbitrary verification code making it possible to break an existing verification but also allows for foreign verification which is not intended.
  Thus a setting like $egGoogleSiteVerifivation which may hold the string seems like the solution to go.

[andru]

Yeah this makes for a pretty serious issue on publicly editable wikis. The next time I have some development time on the project I'll implement this.

[flying-frigate-technologies]

As far as I can tell, this poses very little risk and can easily be mitigated by simply protecting the main page. Further, as I mentioned on the extensions talk page it will impede the extensions use on wiki farms, some of which will oblige to help with local settings changes - though this wastes their and their user's time - many won't.