Get rid of `_categoryGroups`

[neongreen]

• [x] Remove the _categoryGroups field
• [x] Write an acid-state migration
• [x] Remove related endpoints from the old backend, e.g. (renderRoute <//> itemVar <//> "colors")
• [x] Remove getItemHue
• [x] Remove color-related tests, e.g. wd "changing it changes the color"
• [x] Hardcode gray colors in templates, including Guide.Views.Item and item-info.widget; the colors are #D6D6D6 and #F0F0F0
• [x] Remove color-related code from Guide.JS, if present