Added a RA Classic Mod

As @hifi and others suggested this adds a mod to emulate the original gameplay. Basically what #2632 does but for Red Alert inspired by https://github.com/Iran/ClassicRA but merged back into mainline OpenRA making changes to the engine and rules transparent and using switches instead of hardcoding it. The conversion of rules.ini to yaml is copied @iran from ClassicRA 0.1 with minor tweaks to rebase it onto current bleed. Included are the unmerged pull request #2614 and #2602 as well as the official legacy maps converted by @dan9550 downloaded from http://www.sleipnirstuff.com/forum/viewtopic.php?f=83&t=16139 Thanks to JOo for the interface reskin. The progress is discussed at http://www.sleipnirstuff.com/forum/viewtopic.php?f=83&t=16023 as there is still some stuff to do like #2592 or #2058 up to changing the whole GUI layout back to http://redalert1.com as well as doing more research on original rules and behavior, but this is a good start for die-hard legacy fans to work upon and contribute.