[16.0][IMP] web_timeline: Align to top instead of bottom

[houzefa-abba]

This avoids somewhat ugly very large "Unassigned" first row.

See doc on https://visjs.github.io/vis-timeline/docs/timeline/ > "orientation" setting:

Orientation of the timeline items: 'top' or 'bottom' (default). Determines whether items are aligned to the top or bottom of the Timeline.

Since our view always covers 100% height, the "bottom" default setting was producing that large first row.

This could be a per-view setting; I didn't add a setting for now as I believe this top-aligned behavior is always desirable (as far as I could see when showing this around, at least). I can add a setting if you feel it would be needed.

Before

After