Language-ifying the strings in the primary school theme (Bug 1362871)

Currently there is no way for themes to come with their own language
files - but sometimes a theme may have some text it wants to display
that is unique to that theme - like when it has a homeinfo.tpl file

I've added the checks to the lang path for checking a theme's lang file

To use it in a template you need to indicate the section is a theme by
going:

{str tag=stringtoget section=theme.nameoftheme}

and this will look for the lang file in:
./theme/nameoftheme/lang/langtype/nameoftheme.php

Change-Id: I16d5273ee94ad2c066f7de5922253edde325fbed
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>