* FEATURE: Ability to add components to all themes
This is the first and functional step from that topic https://dev.discourse.org/t/adding-a-theme-component-is-too-much-work/15398/16
The idea here is that when a new component is added, the user can easily assign it to all themes (parents).
To achieve that, I needed to change a site-setting component to accept `setDefaultValues` action and `setDefaultValuesLabel` translated label.
Also, I needed to add `allowAny` option to disable that for theme selector.
I also refactored backend to accept both parent and child ids with one method to avoid duplication (Renamed `add_child_theme!` to more general `add_relative_theme!`)
* FIX: Improvement after code review
* FIX: Improvement after code review2
* FIX: use mapBy and filterBy directly
