Handle $wgAllowExternalImages in wiki configuration.

Parse 'allow external image' information in MW SiteInfo API response.
Set allowExternalImages in wikiconfig based on parser test option.
Check wiki configuration before making links to external images.

Bug: 51092
Change-Id: Ic82387a97ad73eab56a0fda798132b88c9ea7ee1
Merge "Handle $wgAllowExternalImages in wiki configuration."