Merge branch 'rename-abilities' into 'master'

Rename abilities to correspond contoller/model action names

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
```

See merge request !896