Dont allow change of MR target or source projects after it was created

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'bug/mr_iids_duplicate' of /home/git/repositories/gitlab/gitlabhq
Point ot 6-4-stable

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Update guide for 6.3 to 6.4

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Version 6.4.0

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>