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.

This generally gives us better behavior:

• Small screens get a scrollbar to see a reasonable amount of content.
• The UI doesn't turn into a total mess if one task has a whole novel of text.

Test Plan:
Old behavior, note that there's no scrollbar and the cell is so narrow it is useless:

{F6320208}

New behavior, same default view, has a scrollbar:

{F6320209}

Scrolling over gives you this:

{F6320210}

On a wider screen (this wide or better), we don't need to scroll:

{F6320211}

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20345