Display overlapping comment bubbles side by side

Review Request #6970 — Created Feb. 22, 2015 and submitted — Latest diff uploaded

Review Board
reviewboard, students
This is the second part in improving the diff comment bubbles.

Multiple comment bubbles on the same range will end up overlapping, making it hard to click or inspect on the comments below. This change allows the overlapping comment bubbles to spread out side-by-side when the mouse hovers above them.
Ran js tests.

Manually testing:
- Added multiple overlapping comment bubbles on the same line to check if the bubbles spread out and collapse correctly.
- Checked tooltips are all displayed on the right side of the range of comment bubbles.
- Checked that clicking the comment bubbles brings up the comment editor.
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