Improvements to delegating discussions in a merge request to a new issue

In gitlab-ce!7180 I implemented a first iteration for moving discussions in a merge request to a new issue based on the discussion in gitlab-ce#23589.

Some things still need to be addressed:

• The warning on the issue form needs to be more visible, @regisF made this suggestion: https://gitlab.com/gitlab-org/gitlab-ce/issues/23589#note_17481832

• The links within the merge request status widget should become buttons as discussed in gitlab-ce#21135, I like the suggestion by @pedroms https://gitlab.com/gitlab-org/gitlab-ce/issues/23589#note_17482095:

• It would also be nice to expose this functionality to the user even when the "Only allow merge requests to be merged if all discussions are resolved"-flag is disabled on a project. @hazelyang suggested adding a button next to the N/M discussions resolved https://gitlab.com/gitlab-org/gitlab-ce/issues/23589#note_17265599

• To allow the user to create a new issue for a single discussion we could add a button next to the resolve button: