Build the node graph frontend

[Keavon]

Done in #581:

• [x] Create a Node Graph panel
• [x] HTML/CSS for the graph
• [x] HTML/CSS for the nodes
• [x] Generating horizontal flow connection curves
• [x] Styling horizontal flow connections
• [x] Generating vertical composite connection curves
• [x] JS for panning/zooming in the graph
• [x] Visualizing grid dots neatly at all zoom levels
• [x] Starting new flow connections by dragging from output ports

TODO:

• [ ] Styling vertical composite connections
• [ ] Solve Z ordering so links are behind nodes, except when entering/exiting a node's port
• [ ] Swap placeholder HTML for data model-driven Vue templating
• [ ] Assisting new flow connections by snapping to hovered input ports
• [ ] Persisting new flow connections by dropping onto input ports
• [ ] Dragging existing flow connections out of their input ports to be moved elsewhere or dropped
• [ ] Starting new composite connections by dragging from output ports
• [ ] Assisting new composite connections by snapping to hovered input ports
• [ ] Persisting new composite connections by dropping onto input ports
• [ ] Dragging existing composite connections out of their input ports to be moved elsewhere or dropped
• [ ] Synchronizing user-drawn links with the Vue data model
• [ ] Selecting/deselecting nodes by clicking and Shift-clicking them
• [ ] Selecting/deselecting nodes by dragging a box around them (with Shift to append to an existing selection)
• [ ] Dragging a(ll) selected node(s), plus nodes that are rigidly attached in a chain (those sharing a port without a link drawn)
• [ ] Snapping the dragged node(s) to the grid
• [ ] Smooth interpolation between grid squares when being dragged so grid snapping feels less jarring (look into using CSS transitions for this)
• [ ] Breaking a rigidly attached chain at a shared port so a link connects the ports
• [ ] Uniting the Vue data model for nodes and link connections with the backend node graph model (or tree-based folder/layer structure if the node graph backend isn't ready yet)