adding comment to "diff view" should have option to create discussion in the parent MR
### Description
adding comment to "diff view" should have option to create discussion in the parent MR
When adding a comment to an MR, the button at the bottom "add comment" has a dropdown side-button with the option to create a discussion.
Discussions, as separate threads, can be "resolved"
When using the comment bubble on distinct lines of the diff view for an MR, the "add comment" button does not have the dropdown.
### Proposal
* add dropdown side-button to the "add comment" button in the diff view of an MR
* automatically add a link from the new discussion to the specific line in the diff view
allows "resolution" of code review comments in their original context
### Links / references
### Documentation blurb
Extremely useful for "in context" code review.
issue