Merge Reaction renderer features, remove "#ifdef REACTION"s
Merge branch 'reactionmerge'
Load pk3 checksums from mod-dir/PAKSUMS

Also allows specifying if each pk3 can be downloaded.
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'

Conflicts:
	code/qcommon/files.c
Merge remote-tracking branch 'upstream/master'