Do not allow work packages table cells to wrap

Limit table cell width to 300px.

Bug # 13498

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Lay out work package view using CSS flexible boxes

This resolves various issues with scrolling, creating 2-3 main
scrollable areas: the main menu, work packages list and, when open,
the details pane.

User Story # 14811

Signed-off-by: Alex Coles <alex@alexbcoles.com>