OEL-1167: Move blocks to root module config/optional folder.

modules/oe_whitelabel_multilingual/config/optional/block.block.oe_whitelabel_main_page_content.yml

-langcode: en
-status: true
-dependencies:
-  module:
-    - system
-  theme:
-    - oe_whitelabel
-id: oe_whitelabel_main_page_content
-theme: oe_whitelabel
-region: content
-weight: 1
-provider: null
-plugin: system_main_block
-settings:
-  id: system_main_block
-  label: 'Main page content'
-  label_display: '0'
-  provider: system
-visibility: {  }

modules/oe_whitelabel_multilingual/config/optional/block.block.openeuropa_content_language_switcher.yml

-langcode: en
-status: true
-dependencies:
-  module:
-    - oe_multilingual
-  theme:
-    - oe_whitelabel
-id: openeuropa_content_language_switcher
-theme: oe_whitelabel
-region: content
-weight: -4
-provider: null
-plugin: oe_multilingual_content_language_switcher
-settings:
-  id: oe_multilingual_content_language_switcher
-  label: 'OpenEuropa Content Language Switcher'
-  label_display: '0'
-  provider: oe_multilingual
-visibility: {  }