Support custom colors

[psychemedia]

We can support custom color schemes using constructs of the following form in empinken.yaml:

- name: empinken_activity_color
  description: color
  input_type: color

The settings are preserved (and can be initialised via) .jupyter/nbconfig/notebook.json.

{
  "empinken_activity_color": "#db1f1f",
  "empinken_pink": true
}

With the extension enabled it thus might end up as something like:

{
  "load_extensions": {
    "empinken/index": true,
    "nbresuse/main": true,
    "jupyter-offlinenotebook/main": true,
    "jupyter-js-widgets/extension": true,
    "nbextensions_configurator/config_menu/main": true,
    "empinken/static/index": true
  },
  "empinken_activity_color": "#db1f1f",
  "empinken_activity_color2": "#d25151",
  "empinken_pink": true
}