Allow multi-line annotations

[chvp]

Allow to add comments/questions on a continuous selection of multiple lines.

From #1852 (comment)

[bmesuere]

Github behaviour:

• if you hover over a line, a plus button appears in the left margin
• if you hover over the button, the button grows in size
• if you click the button, the comment form appears
• if you drag the button, the button stays in place but a thick line in the color of the button appears in the margin
  • you can only drag down, not up
• if you release the drag, the comment form appears. The button and line disappear, but the selected lines are highlighted in yellow. Above the text area, a small help text "Commenting on lines xx to xx" is shown. For single line comments, the highlight and info text is not present.
• when rendering a multi-line comment in the code view, GitHub does not show which lines the comment belongs to. It is rendered under the last line of the selection as a normal comment. In the PR-comment view where just the comments are rendered the additional lines are included in the small code snippet that shows context.

[pdawyndt]

Maybe related:

• comments could be moved to another location by updating the numbers in a help text like "Commenting on lines xx to xx" (both for single line and multiline)
• I often repeat the same comment within the same submission: copy followed by move might speed up this process

[jorg-vr]

completed by #4596