Feature: Add Bootstrap alert to Boost Union settings if Boost Union (or Boost Union Child) is not the active theme

[abias]

It may have happened to you before:
You are working on a Moodle instance with Boost Union installed, are doing some Boost Union configurations, store the settings and want to have a look at the result - but they do not have any effect!

I possible reason, especially on freshly installed Moodle sites, is simply that Boost Union (or Boost Union Child) is not the active theme (yet).

It may be worthwhile to add a Bootstrap alert to the Boost Union settings pages to inform the admin about this fact.
The existing function theme_boost_union_is_active_theme() can be leveraged to easily implement this alert.