Fix 2+ discussions on one line not working

What does this MR do and why?

For #343669 (closed)

When opening a second discussion on one line, the second comment box fails to open (and also leaves the UI without the option to open another discussion on that line).

This MR resolves that problem by making sure the discussion form always has a range.
In the context of the second note form, that range will start and end at the attached line.

Screenshots or screen recordings

Before	After
simplescreenrecorder-2021-10-22_15.18.28	simplescreenrecorder-2021-10-22_15.17.53

How to set up and validate locally

1. Leave a comment on a line of code.
2. Click the box labeled "Start a new discussion..."

MR acceptance checklist

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