Initialize theme blocks on creation of theme mapping

Problem/Motivation

Currently Block Theme Sync acts only when a block is saved. Because of this, a theme may lack blocks that are present for the source theme.

This issue is complicated by a quirk of Drupal core. By default, when you install a theme, if the theme doesn't supply any blocks of its own, Drupal core copies over the blocks that are enabled at that time for the current default theme. So, if the theme doesn't provide any blocks, it will get an initial set of blocks created, possibly from the source theme--but possibly from some other theme that was default. If it does provide blocks, it won't get any from another theme.

Proposed resolution

Extend our config event subscriber to conditionally sync blocks when a theme mapping is created.

Remaining tasks

User interface changes

API changes

Data model changes