Allow alpha channel for colors in settings form.

Problem/Motivation

The settings YAML can be edited to have alpha channel in the colors. But this cannot be done from the UI.

Example:

colors:
  main: '#10857f88'
  secondary: '#ffffff'

Steps to reproduce

Import the above config, then visit the settings form. The "main" color will show as black, despite it working with alpha in the UI when editing content's layout.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes