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 adding switches instead of hardcoding it. The conversion of rules.ini to yaml is copied from ClassicRA 0.1 release by @iran 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.