Refactor `ContentBlockController::save()`

Refactor `ContentBlockController::save()` to account for possible
overrides. Also account for future additions to code.
Remove unneeded `Loader::model('file_version')`

Remove unneeded `Loader::model('file_version')`

concrete5.6+ covers this with autoloading
Merge pull request #1048 from mkly/remove_uneeded_model_load

Remove unneeded `Loader::model('file_version')`
Merge pull request #1047 from mkly/refactor_content_block_controller_save

Refactor `ContentBlockController::save()`