2770: terminate on SIGTERM.

When SIGTERM is received, the Keep server shuts down gracefully. (refs #2770)
2753: Make Collection filenames with existing helper.

This is pure refactoring to make the code more consistent.
2753: Add tests for CollectionsHelper.file_path.
2753: Add Workbench Collection#files_tree.
2753: Workbench renders "0 bytes" for empty file sizes.
2753: Workbench shows Collection files in a tree.
2753: Replace Workbench file tree backgrounds with hover effect.

Based on feedback from Tom in refs #2753.  I'm still concerned about
users being able to line up buttons with filenames, so the highlight
on :hover is intended to help with that.
2753: Refactor Collection view to use library CSS.
2753: Move file tree CSS to collections.css.
2753: Add space between Workspace file tree buttons and scrollbar.
Merge branch '2753-collection-tree-view'

Closes #2777, #2828, #2753.
Merge branch '2770-keep-sigterm' (closes #2770)