User Properties: Introduce consistent way of getting user properties.

Adds a check that the user object exists and standardises the approach.
hexes.js: Make the hexes helper reactive

Previously the helper was non-reactive, and this can cause issues with conditional rendering of the hexes.
User Properties: Add new properties sp_show_coords and sp_show_minimap

These properties will replace the equivalent session variables located in the settings panel.
User Properties: Replace session variables show_coords and show_minimap with user properties