(bug 45387) (bug 45388) $wgRSSAllowImageTag new name; bug fixed

+++ breaking change +++
variable name $wgAllowImageTag renamed to $wgRSSAllowImageTag

+ fixed a problem that $wgAllowImageTag was not obeyed
+ added $wgRSSAllowImageTag to the variable declaration list in RSS.php

Change-Id: I112ebcdf492dfe80ba187fe6ab2406d3eed10f6a