Improve copy in mr warning messages

Coming from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3669

Problem / Proposal

1

This looks weird, can we add "into target branch [branch name]"

Screen_Shot_2017-12-15_at_00.44.14

Screen_Shot_2017-12-15_at_00.56.52

2

"please resolve these discussions" is unnecessary and we don't do the same at WIP

Screen_Shot_2017-12-15_at_00.51.12

Screen_Shot_2017-12-15_at_00.51.40