Frontend for the node graph

[Keavon]

Done in #581:

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

Done in #846:

• Swap placeholder HTML for data model-driven Vue templating
• Assisting new flow connections by snapping to hovered input 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)
• Selecting/deselecting nodes by clicking
• Displaying parameters in the Properties panel

Done in #855:

• Synchronizing user-drawn links with the Vue data model
• Persisting new flow connections by dropping onto input ports
• Multi-selecting nodes by Shift-clicking them

Done in #863:

• Secondary inputs
• Dragging nodes
• Snapping the dragged node(s) to the grid

Done in #866:

• Exposing inputs to the graph

Done in #869:

• Node add menu
• Search box in add menu

Later (post-MVP) improvements:

• Solve Z ordering so links are behind nodes, except when entering/exiting a node's port
• Selecting/deselecting nodes by dragging a box around them (with Shift to append to an existing selection)
• Smooth interpolation between grid squares when being dragged so grid snapping feels less jarring (look into using CSS transitions for this)

[Keavon]

Closed by the various PRs listed in the issue description during Sprint 20.