Feature: Extended user theme customization support

[billz]

This adds two new settings to config.php to enable dynamic theme customization.

define('RASPI_BRAND_ICON', '');
define('RASPI_BRAND_COLOR', '#2b8080');

For example, a Switchberry theme may be applied simply by adding the following:

define('RASPI_BRAND_TEXT', 'Switchberry');
define('RASPI_BRAND_ICON', 'fas fa-clock');
define('RASPI_BRAND_COLOR', '#df7618');

This renders a gradient background and form elements on the login template from a single color value:

The remaining templates are themed with the selected fa icon and brand color: