Increase $wgSVGMaxSize to 5120 pixels wide (previously 2048).

Increase $wgSVGMaxSize to 5120 pixels wide (previously 2048).

People use mediawiki for all sorts of things, restricting to
2048px wide seems much to strict as the default setting (imo).

It should be noted, MediaWiki, can, in the default config
(but with some non-default user prefs), include images of
up to size 2560 px on the image description page, so the max
svg size should at least be that big.

Change-Id: I306a792e61e7c738982b9dbde8a63aa31acf38a8
Note: this doesn't affect WMF, as they have it overridden to 4096