Allow asset compilation without `color_schemes` table

Contribute Feature
4

Heroku doesn’t run migrations for you, so you have to push to Heroku first and then run migrations. However, Heroku will reject your deployment if asset compilation fails, so you get ‘stuck’.

For others in this situation, I was able to get around it with a few changes. In line 51 of discourse_sass_importer I skip the ColorScheme.enabled check:

    # OVERRIDE FOR HEROKU COMPILATION
    if color_scheme = false#ColorScheme.enabled
      ColorScheme.base_colors.each do |name, base_hex|
        override = color_scheme.colors_by_name[name]
        contents << "$#{name}: ##{override ? override.hex : base_hex} !default;\n"
      end
    else
      special_imports[name].each do |css_file|
        contents << File.read(css_file)
      end
    end

And then in 106 of discourse_stylesheets.rb:

  # theme = (cs = ColorScheme.enabled) ? "#{cs.id}-#{cs.version}" : 0
  theme = (cs = false) ? "#{cs.id}-#{cs.version}" : 0

I would love to have a SiteSetting to disable ColorScheme without checking the DB. That would still feel hacky, as I don’t really want to disable ColorScheme, I just want it to fail silently.

I’d be enthusiastic about contributing code for this solution if it’s agreed that it is a good idea.