Get "Merge when build succeeds" to work when commits were pushed to MR target branch while builds were running

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

This will fix merge problem for merge requests between forks created
before 8.0

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

This reverts commit 5daf44b7, reversing
changes made to 2f706fbd.

This reverts commit 957e849f, reversing
changes made to 6b9dbe9f.

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

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

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

write_ was renamed to create_
modify_  was renamed to update_

So now in update action we have next code

def create
  can?(current_user, :create_issue, @issue)
end

def update
  can?(current_user, :update_issue, @issue)
end

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

* store @participants in variable
* store result of subscribed? call into variable

In total it reduce amount of SQL queries for merge_request with 10
comments/participants twice.

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