R1:e586ed439a79

Improve overflow/ellipsis behaivor for very wide task graphs

Summary:
See downstream <https://phabricator.wikimedia.org/T171648>. The `T123 Task Name` column in graphs can currently fold down to 0 pixels wide.

Although it's visually nice to render this element without a scroll bar when we don't really need one, the current behavior is excessive and not very useful.

Instead, tweak the CSS so:

- This cell is always at least 320px wide.
- After 320px, we'll overflow/ellipsis the cell on small screens.