Bug: Make outside-left and outside-right block regions really responsive

[abias]

This is a follow-up issue of #30 / #118

Currently, the outside-left and outside-right block regions are marked as experimental due to the fact that they are displayed fine on small and wide screens but do not wrap properly on medium width screens:

Wide screen:

Small screen:

Medium width screen:

Notification in settings:

This is due to the fact that the outside-left and outside-right block regions are currently built as inline-block elements instead of flex elements which couldn't be solved during the PR peer review without blocking the integration for fixing and testing substantially longer.

This issue should be fixed and the "experimental" notification should be removed from the settings page then.