External links should be marked as such in HTML

[NABN00B]

We should mark links that lead to external websites as such by applying the external class and by setting the attribute rel="external".

Links to the following should be exempt:

• (www.)ppsspp.org or dev.ppsspp.org (depending on environment)
• builds.ppsspp.org

[hrydgard]

Good idea, we can do this automatically in the rust code, I guess after html generation..