Sidebar - replace tree component with ag-Grid #91

The 3rd-party tree component we use on the sidebar, while nice looking, is slow and doesn't scale well with larger trees.

Instead, let's replace it with ag-Grid in tree mode and we can load the nodes in a flat structure, rather than nested, using a tree num system.

Here, also, add the ability to filter by node type (database, code snippet, file box, &c.)

Decided to do a custom implementation, since tree-listing in ag-grid is heavier than we need, and requires enterprise-edition.